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Use of RO, DI, Softwater in Aquariums


By Carl Strohmeyer-PAMR 40+ years experience
Updated 8-5-23

INDEX of this article:

Which system should an aquarium keeper use, RO, DI or both OVERVIEW:

While the use of Reverse Osmosis and/or De-ionized water has been around for quite some time, their use has exploded of late, often for good reasons, but unfortunately there is a lot of miss-information about how these systems work (including in a popular YouTube video) and gimmicky products that over sell the less experienced aquarium keeper.
I have used both types of equipment over the last few decades, and this includes variations within such as the use of Cellulose acetate membranes which are the oldest form of commercial RO membranes, yet still used along with lower quality TFC membranes, in most RO and RO/DI combinations to keep the price MUCH lower!

Re-mineralization after use of RO and/or DI water and essential minor and trace elements/minerals is also discussed.

I also address the use of water that has passed through typical home/office water softeners and why the use of such water poses a serious long term threat to ANY fish' health.

Here is a basic video about this subject:
Do I Really Need Aquarium RODI
Do I Really Need Aquarium RODI? With A Catch...



Reverse Osmosis System

RO/Reverse Osmosis water is simply water that is run under pressure through a micron pre-filter, then a carbon pre-filter & finally a osmonic membrane.
For reef keepers, these are often a must and at a cost of just pennies per gallon, often well worth the investment over paying .25 cents per gallon.
For Discus keepers and similar biotope aquarium RO or RO/DI units can also be helpful.

The membrane is the most expensive part of a RO unit and is rated in gallons per day (gpd).
With water that is too hot or cold (the ideal operating temperature is between 70-80 F or 21-27 C), along with low water pressure (psi), and high TDS (total dissolved solids) all will combine to slow the rated capacity of a RO Unit & membrane. High TDS will also affect the life of the membrane.
Often the pressure provided by the tap water is adequate, however with high capacity RO Units additional boost pumps are required (usually units over 100 gpd which require 65 to 80 psi).

The few top quality aquarium RO units still produced use TFC Medical Grade (thin film composite, sold by Pentair USA) membranes which is much better for removal of ammonia, phosphates, herbicides, pesticides, and TDS.

Other units still utilize the popular Economy TFC membranes sold by Dow under the Trade Mark of FilmTec. This includes the Coralife, which I have considerable experience with along with Bulk Reef Supply & many others.

The other common type is the CTA, a.k.a. the cellulose triacetate membrane (also sold by Dow & others) which is used in low cost RO units commonly sold by discounters such as Amazon. Cellulose acetate membranes are the oldest form of commercial RO membranes and the lowest cost to produce, hence the much lower costs of systems utilizing these.

For those thinking they may have gotten a good deal on a RO system that claims to use a TFC membrane, I will remind readers of basic economics and that is while many sellers will price a system low to get a customer and then make the money on parts (think about the cost of HP printers, then the cost of replacement ink), if the replacement parts are at or near the cost of a top quality TFC membrane, then you do NOT have a unit/system utilizing a top quality membrane.
I have used many different RO units and the the results (before passing through a DI chamber) in higher waste water, higher TDS and other measurable aspects such as GH are notable.

Below is a screen-shot from the manufacture/distributor order web site of the cost a retailer will pay for these items Pentair membranes. If the replacement retail price is at or near these prices, then unless it is a clearance item, it is not a high quality TFC membrane (simple economics 101):
Actual wholesale cost of a TFC RO membrane

I, along with the person who purchased my aquarium maintenance, research, and design company have used the Coralife RO and RO/DI systems. While they claim to use the TFC, our results were not the same as with other units that cost considerably more for the membrane, which again brings up the argument that I can purchase wholesale a 100 GPD Coralife membrane for $25 (or less) but the better Pentair USA medical grade membrane used by several high end RO and RO/DI systems have a wholesale cost of $50.

Keep in mind too that FilmTec is a Dow Trademark and does NOT mean this is a TFC Membrane or if it is, the best TFC since price determines quality here.
The Filmtec can be either the TFC or CTA. The Dow Economy FILMTEC FT30 Membranes are TFC, however many models are not, so make sure to do your homework and apply common sense logic as per economics whether it is the CTA, TFA or a better medical grade TFC such as the Pentair USA TFC Membrane.
Example Reference: Filmtec Cellulose Triacetate Membrane

Premium Aquarium RO System in operation showing efficiency of micron fiber
A test of an RO or RO/DI system/filter is if the reject water is higher than three gallons of water for every one gallon I would question the quality of the membrane used. This can result in concentration of ammonia as I have noted in experiments in the past.

Also be aware that CTA membranes lower permeability requires higher pressures so booster pumps may be needed for optimum efficiency.
This is especially true if you use a RO system that uses twin membranes as a way to increase output while keeping the purchase cost lower than would be the case with a higher GPD membrane.
Even with a TFC membrane (regardless of membrane quality), I do not advise twin membrane systems as this increases the pressure needed to run the system, often complicating the system or lowering efficiency if a booster pump is not added.

Both membranes are damaged by chlorine, which is one reason for the carbon pre-filter (canister).
The membrane should be changed every 2-3 years or 12,000-18,000 of total water produced (whichever occurs first).

A properly functioning membrane will work effectively after a 2-3 gallon break-in (throw this water out), after this initial start up, you should have 1 gallon of "good" water for every 3 gallons of reject water. If higher, this can concentrate molecules that the RO membrane cannot remove (which is why spending a few extra $$ for a good RO unit such as the TMC designed specifically for aquarium use is worth it, not looking for the best deal via eBay, Amazon, or popular Aquarium Supply discounters).

Aquarium RO, RODI TDS ResultsTo the right is proof that the quality on many levels of the RO system affects simple RO results.
Here you see a high TDS before and ZERO TDS after WITHOUT EVER USING A DI Chamber!!
This is using the ultra premium AAP/TMC RO System.

RO System Product Resource: Tropic Marine Centre V2 Pure Reverse Osmosis System using Pentai USA Membranes

TDS Monitor Resource: TMC Total Dissolved Solids Monitor

Generally speaking, a well maintained RO unit will reject about 97%+ of TDS.


*DI Unit; which stand for "De-Ionized".
Most commercially available DI units are simply a RO unit with an additional DI canister/chamber following the RO membrane (API makes a basic DI only unit though). This chamber utilizes resins that further remove TDS from water.
With this system, you can achieve 0 mineral cations and a TDS of 0 after starting with tap water over 250 TDS, while this same tap water run through a RO only unit would generally result in about 7 TDS at most with a quality RO only system (I average only 2-3 TDS with the "top of the line" AAP/TMC RO System).
You will also achieve a pH of 7 since a DI unit or chamber affects ionization where as a RO by itself does not.

For my purposes I found that a well designed TFC (thin film composite) RO only unit was/is more than adequate, however if you are making water for extremely sensitive purposes (such as a car battery), or have high ammonia levels in your tap water, a DI/RO unit may be the better choice for you over a RO only unit.
The reason is much of what the additional DI chamber/resin removes is mineral cations as well as all charged molecules (electrolytes).

Which is why DI water is better for a car battery, but these few mineral cations missed by RO only units are not a problem for any freshwater or saltwater aquarium (unless run off from a Home/Office Water Softener) and in fact these ions are generally beneficial.
As well the operating costs of DI chambers are very high as I have had to replace the resins in these chambers as frequently as once per week with some units under high use.

API Tap Water Filter Basic De-ionization filter
Another alternative is the "API Tap Water Filter" which is nothing more than a large disposable de-ionization resin filter with built in carbon and filter floss. With this filter you basically throw way 90% of the filter when exhausted.
This filter does not utilize any other chambers or RO membranes of any type.
The API Tap Water De-Ionization Filter is a very economical way to purchase such a filter, but very expensive long term. I have used these and found the chamber is only good for about 100 gallons of water, which is not a bad deal for a Betta keeper desiring to mix pure water or cut their tap water, but it makes little sense for a Reefer, Discus Keeper, or Planted Aquarium Keeper that needs volumes or water.

The bottom line is to NOT believe the some of the hype some sellers of Four to Six Stage RO/DI units put forth and save your money (both initially and ongoing) and purchase a QUALITY TFC RO only water system which WILL result in 0 phosphates, 0 nitrates, and chloramines along with very low TDS readings with less reject water and lower operating costs.
I will also state that these Four to Six Stage RO/DI systems are certainly not bad either, only that why spend the money, including ongoing DI resin replacement for a good RO/DI system when a better RO system is all you need??

There is a good reason some of these 4-6 stage RO/DI units can be sold at prices comparable to or even lower than top quality TFC 3 stage RO systems and and this is the use of (consider these points before falling for the gimmicks):

What you will get is a long term higher operating cost since replacement resins are not cheap, and again in most aquarium applications, not necessary as per my own long term and extensive use and testing!

As noted earlier, another test of an RO or RO/DI system/filter is if the reject water is higher than three gallons of water for every one gallon I would question the quality of the membrane used. This can result in concentration of ammonia as I have noted in experiments in the past.
Often these economy RO/DI systems sold by popular discounters will have higher reject water.

There are really only two MAYBE reasons to purchase a unit that also features de-ionization:

  1. If your tap water has high ammonia or is in a municipality that uses chloramines (such as some SW USA locations). An expensive to operate DI chamber are often are only good for about 50 gallons and make little difference for chloramines if well designed (some of the cheap Amazon/eBay units are not).

    If the RO unit is well designed using quality catalytic carbon (such as the TMC models), the fact is these will remove chloramines.

    *Removing Chloramines from Water
    * Catalytic Carbon for Chloramine Removal

  2. You want to run your system directly off from a home or office water softener that uses sodium chloride or potassium chloride, since these systems radically alter the mineral ions in ways detrimental to fish and invertebrates (reference section about use of home water softeners later in this article).
    The use of an RO system only will not remove much of the sodium ions that can be detrimental to your aquarium health (which includes osmotic function of fish).
    In fact this is a case where by the use of an RO/DI system, operating costs would actually be lower than an RO only that is run off from tap water or well water.


This is another aspect of misinformation is what is removed by a particular component of an RO or RO/DI system. This misinformation is even spread by one of the largest sellers online of large RO/DI systems that are often not necessary for aquarium use, both freshwater and salt water, but based on this misinformation, many are unfortunately convinced that they need these additional components.

This list below is based on a properly functioning RO/DI system, as often additional modules of water purification are needed if one module/component does not completely remove undesirable elements of input water such as nitrates.

  1. Sediment/Micron Pre-Filter;
    The main purpose of this chamber/component of a RO/DI system is to remove sediments such as rust from the water.
    From my experience, these sediment filters generally range from 1 to 20 micron in pore size.
    These should be changed frequently to maintain pressure for the rest of the unit as well as extend the life of the membrane. How often you need to change it can vary greatly.
    What my experience has shown me that changing of the micron cartridge has little bearing on the life of the resin in a the DI chamber (if a DI chamber is even used).

    If the water coming in to the RO or RO/DI system contains a lot of sediment such as rust or large sediment particles, multiple stages or better; a separate sediment system is best added (rather than make the mistake of purchasing multiple stages of filtration after the sediment filter).
    Adding a 20 micron pre-filter (purchased at Home Depot, etc.) to your existing RO system can help considerably. Or even better, adding a whole house system such as this one to remove rust, iron, etc.

    The above is a more cost efficient solution when one is looking long term at water that is high in TDS and sediment rather than the 6 stage or larger RO/DI systems sold by popular aquarium supply retailers that do not properly address the cause of such issues with stepped down micron filters.

  2. Carbon Filter;
    When a good quality catalytic carbon filter block is part of your RO system, it removes many chemicals, toxins, chlorine and YES chloramines assuming a quality catalytic carbon filter block is used in the system.
    Unfortunately this aspect is missed in a popular YouTube video promoting more elaborate systems

  3. Reverse Osmosis (RO) Membrane;
    Properly functioning Reverse Osmosis (RO) Membrane for aquarium use I have already gone into great detail what makes a good membrane, so I am going to assume an optimum quality membrane here.
    The main purpose of the membrane is removal of mineral salts, which at the proper water pressure and temperature an optimum membrane is VERY efficient at doing.
    This same membrane can also remove most nitrates and ammonia. In fact my own tests over the years showed 100% removal of nitrates.
    However some claim nitrates can get through, but this again falls back partly to the membrane used, as my results varied with systems such as the Coralife RO system using the FilmTec membrane which is also used in many popular RO/DI systems, did in fact leave some residual nitrates while the TMC V2 system had 0 nitrates.
    I should note that these results may also be part of the system design too such as how the system maintains water pressure to the membrane.
    It should also be noted that any membrane is only as good as the sediment and carbon pre-filters.

    The picture to the right shows how a properly functioning RO membrane rejects sodium ion while allowing water molecules to pass.

  4. De-Ionized (DI) Module/Chamber;
    A DI resin chamber traps all charged molecules passing through it, and leaves uncharged (neutral) molecules free to pass through.
    Water, for example, passes through it, as would other uncharged inorganic molecules such as oxygen (O2). Ions and minerals salts missed by the carbon & some RO membrane such as sodium (Na+), copper (Cu++ or Cu+), ammonium (NH4+), phosphate (PO4---), silicate (Si(OH)3O-), and acetate (CH3CO2-) generally get caught (assuming the resin is not depleted).

    The result is usually pH neutral water unlike the water coming from even the best of RO only systems.
    The DI resin chamber can also be a good back up from nitrates, chloramines, and in particular ammonia/ammonium missed by the earlier chambers.

Further References:
Reverse Osmosis/Deionization Systems to Purify Tap Water for Aquaria
Removing Chloramines from Water


Another method to prepare DI Water is to distil the water by boiling then collecting the condensation of this water (AKA distilling).

This can be an economical DIY project or there are commercial water distillers also available.
If you are making a DIY Water Distiller, make sure to use condensation collection coils & other related surfaces that will not add elements back into the water, especially copper coils (which can defeat the purpose of the water distiller).
Some plastics can also add chemicals back into your water, generally stainless steel is accepted to be the best surface to use for condensation collection.
Please click on the picture to enlarge

Betta Tank with RO Water, Correct Use Use of RO, DI (Distilled) water in Aquariums or Betta Tanks;

Do NOT use 100% RO water unless you are an advanced fish keeper with time on your hands, which includes having GH and KH Test Kits/Strips, as well as some basic knowledge or aptitude of chemistry.

Generally the use of RO or DI water in freshwater aquariums should be restricted to blending with tap or well water so as to "Cut" the water resulting in lower buffers and hardness of aquarium for use with Amazon River, Southeast Asia (such as Bettas), or similar fish.

Generally I start with 25% RO water and work up from this over time (if necessary).
The reason is that RO and similar water is NOT properly mineralized for correct osmoregulation with essential minerals such as calcium nor is there any carbonate buffers to maintain a stable pH which the lack there of would result in a roller coaster pH in the aquarium, often with disastrous results.

That said, for advanced freshwater fish keepers who desire exact Amazon River or SE Asia aquariums (or even Goldfish, Livebearers, etc.);
The use of 100% RO water can achieve phenomenal results providing all trace and major minerals as well as buffers are replaced (buffers are very often missed with users of 100% RO Water).
The reason is that you can reproduce the exact water conditions you desire (assuming again you are familiar with water chemistry), without starting from a point of incorrect minerals or even high nitrogenous organic compounds such as Nitrates (which are often found in tap water or well water).

Everything you need to know about water chemistry:
Aquarium Chemistry

With Marine Reef Aquariums, since most better salt mixes are exacting in their mineral and trace element blends, the use of RO water provides for better results.
More importantly, topping off for evaporation in marine aquariums with tap water (even fish only tanks), results in climbing nitrates, sometimes falling alkaline reserve, incorrect usable calcium levels, and more. So the use of RO or DI water is often a must in marine aquariums for evaporation from my experience.

Correct use of RO Water:

First, if small amounts RO Water (under 25-50%) is used to "Cut" hard/alkaline tap/well water, often only electrolytes with mild buffers already added such as SeaChem Replenish is all that is necessary, not strong buffers or baking soda.

For those considering using RO or DI water in higher amounts, it is important to note that for proper osmotic function trace amounts of several minerals are required.
Many of these supplied simply by water changes and supplements such as Wonder Shells.
For this reason pure RO (Reverse Osmosis) or Distilled water are not good for water changes unless re-mineralized (with products such as Replenish, Wonder Shells, and Buffers) or blended with tap or well water that is possibly too high in many minerals (a very high GH over 500 ppm can slow respiration in some freshwater fish).

To use with RO water:
*Aquarium Mineral Blocks
*SeaChem Replenish; water RO, DI mineral, carbonate replenisher
*SeaChem Aquarium Buffers

For Alkaline/Acid Buffer Ratios for use in Planted, Amazon, or SE Asian Aquariums (softer, low pH aquariums), please see the chart below (based actual ratios, NOT dosages):
Diagram for Alkaline and Acid Buffer mix in freshwater aquarium

*Please note that the above SUGGESTED Ratios are based on 100% pure water and in most instances the water used is not, especially if the Water is from a RO Filter that is not serviced regularly, or is simply a poor quality RO Filter (often one intended for human use, not aquarium use).
The bottom line is to start with these ratios, then adjust the rations until desired parameters are met; then write down these rations and use them in the future.

*As well as noted this is for low pH/softer water aquariums!!
For goldfish, livebearers, or even general community tanks, the use of Malawi Buffer (or Marine Buffer) is advised. Also the use of Acid Buffer as a counter buffer for equilibrium is generally not necessary.

*Do NOT use these ratios to adjust well, tap, blended, or established aquarium water!!
For this you need to add buffers as per aquarium parameters and then find a "Sweet Spot" where your aquarium maintains the desired parameters without a pH roller coaster. This often takes small doses until this is achieved.

*Finally these ratios are based on a starting measurement as per your tank size & suggested amount as per the buffer used (see your product instructions).
In other words if for your size aquarium 1 teaspoon is required of a certain buffer this becomes "1 Part" and then you would use 1/2 teaspoon if ".5 Part" of a counter buffer is also called for.

Adding Blended, RO, or RO/DI back to the Aquarium (Water Changes)

Once your KH, GH, pH, TDS, etc. are where you want these parameters, you water is reading for adding back to the main aquarium. A 32 gallon RubberMaid or similar trash can works for adjusting these parameters prior to addition after a water change (clean first with salt water before using).
If you are looking to have a flow through system, evaporation top off, or similar, I still suggest adjusting in this separate container, then pumping either slowly or quickly from this container.

I personally used two containers, one for simply filling slowly with RO water, then the next one is where I move this water to adjust and/or blend. I move the water to the second container when it is nearly empty, this way I have a constant filling of the first container while the second container is adjusted, then used.
This method still works well if the second barrel is used in a "constant slow water change". I would simply turn off the slow or demand pump while adjusting the water, then back on when complete (which should not take long).

Products that can aid in re-mineralizing RO water (many can be/should be used with other products for complete re-mineralization that includes carbonates)

Basic Electrolyte Replenishing Products (& Links):

*SeaChem Replenish;
Similar to Kent RO Right and API ElectroRight as a primary trace element/electrolyte replenishing product, although this product does add some buffers. Because of the mineral salts contained in this product, it is a good choice in particular for use in softer water aquariums as it does not as readily raise hardness. Replenish has very little affect on pH.
I recommend using this product to add to RO/DI water whether used in full or to "cut" tap/well water. Use the directed amount for the amount of RO or DI water used, not the full gallonage of the aquarium.

*Kent RO Right;
This is for basic trace elements, electrolytes, this product does NOT add most necessary buffers and most major minerals that are especially necessary for general freshwater fish tanks, especially livebearers, African Cichlids, and Goldfish.

*Aquarium Pharmaceuticals Electro Right;
As with Kent RO Right, this is for basic trace elements, electrolytes, this product does not add most necessary buffers and most major minerals that are especially necessary for general freshwater fish tanks, especially livebearers, African Cichlids, and Goldfish.

KH Buffers & GH/Mineral Additive Products (& Links):

*SeaChem Alkaline Buffer and SeaChem Acid Buffer
These to products above are for balanced carbonate/bicarbonates for correct KH & pH.
These are generally used for planted or low pH/soft water aquariums.

*SeaChem Malawi Buffer (or SeaChem Marine Buffer);
This would be the buffer of choice (used as directed) for Livebearers, Goldfish, African Cichlids, and similar higher pH/KH fish. I suggest using 1/2 the recommended amount in your RO water prior to addition to your aquarium then add to this until the desired KH is reached.

*SeaChem Cichlid Salt
Similar to SeaChem Buffer along with added sodium chloride salts

Wonder Shell for aquarium*Wonder Shells (Large) An important compliment mineral replenisher; mixes well with Electro Right, Replenish, RO Right, SeaChem Buffer or the similar but much less easy to use, SeaChem Equalibrium.
The Wonder Shell is an excellent compliment to follow up with Replenish or other initial RO Water Electrolyte additives, so as to constantly maintain essential mineral cations.
I recommend to use 1/4, 1/2, or a normal size Wonder Shell based on the type of aquarium kept & mineral needs. This goes for 100% RO/DI regenerated water, RO/DI cut water or 100% tap/well water.

*SeaChem Equilibrium;
Similar to Replenish, Wonder Shells or AragoMight, but with much more emphasis on Potassium and thus planted aquariums.
In most cases I have found this product NOT the better choice when compared to the use of Replenish along with 1/4 to 1/2 dose of Wonder Shells.
WHY? My extensive use of Equilibrium has not achieved as good of results, even in planted aquariums since it does not maintain the mineral Cations required in a closed system nor is it as useful for RO water use. AS WELL, in its use in certain conditions such as water softener water, it can occassionally percipitate out certain mineral Cations in a way that can cause a sudden fish die off.
I realize that that other professionals have a difference of opinion with me here, but I stand by my results and have always used & sold what I had the best results with, so while I really like SeaChem products and think this too is a good product, I have had better results using Wonder Shells, which is why I currently no longer sell this product.

These products can be used separately or together.
The API ElectroRight, Kent RO Right only adds important trace minerals and no carbonates and is usually inadequate by itself.
I generally would soak a Wonder Shell in RO water and then add Buffers (often both acid and alkaline for correct balance) or Cichlid or Marine Salt to further replenish major and minor elements as well as carbonates (as Wonders Shells have little carbonates).

Drinking Water/ Rain Water;

Also be real careful with water label “Drinking Water” (products such as Aquafina are simply RO water that has some minerals added for flavor for human consumption) as this is usually just RO water with a few minerals added for taste and does NOT have the electrolytes needed by fish. Usually pure spring water does have the proper electrolytes needed by fish.

I have seen many (especially on the internet) recommend rain water be used in place of tap water or similar, the reasoning is similar as with RO water that this water is more pure and therefore more healthy for the fish.
HOWEVER this reasoning is severely and dangerously flawed as rainwater generally has little or NO essential minerals for osmoregulation and as well as buffering capacity at all for maintaining a stable pH (rainwater tends towards the acid side on the pH scale) which will result in a rollercoaster pH in the aquarium.
Please note that the pH scale is logarithmic meaning a 1 point change up or down is equal to a tenfold increase in acidity/alkalinity.
Reference: Aquarium Chemistry; pH

Softened Water; Home/Office Water Softeners Use:

Home (or office) water softeners that employ salt (either sodium chloride or potassium chloride) should NOT be used for supplying aquarium water, as these strip most important minerals all the while increasing sodium to very high and out of balance levels (sodium is only required in trace amounts for most fish). The sodium that is present continues to strip ESSENTIAL calcium and other mineral ions. Running an RO system after a home water softener will not solve the problem of these odium ions.
This can severely affect osmoregulation in fish, especially many fish such as Loaches that normally prefer more soft water.
However these same softwater fish do not prefer soft water containing an unbalanced mineral content that is high in sodium but missing other essential mineral ions, which water from a home softener would be, regardless of whether you add back minerals such as the use of a Wonder shell or not!

The use of soft water from sources that utilize sodium OR EVEN from aquarium conditioners that have sodium bases is that the sodium often drives out the essential mineral cations.
The proof is the ability to maintain a good KH, however an un-naturally low GH often results.
One such test I conducted showed a KH of 200 ppm while the GH was only 20 ppm.

A review of Aquarium Water Conditioners:
Aquarium Conditioners, Information

The result is nearly NON-existent ESSENTIAL calcium, magnesium, and other positive mineral ions.
This can have severe affects on all fish, but is an ESPECIALLY noteworthy problem in Goldfish, Livebearers, and Rift Lake Cichlids!!

AS WELL, the use of Softened Water in certain conditions, such as some instances of combing with SeaChem Equilibrium, use of softened water will percipitate out certain mineral Cations in a way that can cause a sudden fish die off.

Another aspect is that softened water contains enough sodium to actually irritate a fish' epidermis, which in particular is harsh for scaleless or smooth scale fish (such as Loaches, many Catfish, Eels, Elephant Nose, and Ghost Knife Fish).

Although likely obvious to most saltwater aquarium keepers, the use of water softener water should NEVER be used for mixing saltwater or topping off a marine aquarium for evaporation.

See also:
Tap water for my Aquarium or Pond; Sodium

Here is how a home or office water softener works:
When the resin becomes saturated with calcium and magnesium, it is regenerated by passing an NaCl or KCl brine solution through the resin. The high concentration of sodium ions in the brine causes the reaction to be reversed with sodium (or potassium if KCl is used) replacing the calcium and magnesium ions which are then discharged into the waste water.

This sodium or potassium that is used to drive out mineral Cations of calcium and magnesium remain in the water, even if in small amounts. These sodium ions are enough to continue to drive out a considerable amount of essential mineral Cations so that simply adding these back via SeaChem/AAP Replenish, AAP Wonder Shells, etc. results in precipitation of these essential electrolytes.
Simple tests with an AAP Wonder Shell bear this out as the Wonder shells dissolves in a pile of dust, but never actually changes the water chemistry!

Here is the equation/formula:

Water Softener Reaction formula, equation


Necessary Minerals

From Original Article:
Do Fish Drink; Osmoregulation in Fish

Here are a few necessary TRACE elements/minerals (electrolytes) and their function.

For a related post that deals with trace elements:
“Plaster in Paris in Aquariums and Ponds”

It is important to have a proper Redox Potential which describes the ability for the loss of an electron by a molecule, atom or ion to the gain of an electron by another molecule, atom or ion.
Without this reducing Redox Potential many minerals cannot be absorbed and properly assimilated. So it is very important to keep a “positively charged” aquarium or a Balanced Redox Potential via proper dissolved oxygen levels, calcium and other electrolytes, proper cleaning procedures and water changes (UV Sterilization can help too).

For more information about the Redox Potential, please see this article:
*The Redox Potential in Aquariums (& Ponds) and how it relates to proper aquatic health (AVOID scam articles on the subject of Aquarium Redox such as by Aquariumscience that are only attack pieces and have no basis in practical experience)

For further reading on this subject, here are a few articles I recommend:

This other Aquarium Answers article is an excellent compliment:
Tap water for my Aquarium or Pond? From Chlorine and Chloramines to Phosphates & TDS

For more aquarium information, please visit this site:

*Sodium Ions:

Carl Strohmeyer- Copyright 2023

Other Recommended Reference & Product Sites

Aquarium Information
Aquarium Information and Resources (Pond too)

Fish Diseases, How to Treat Sick Fish
Fish Diseases | How to Treat Sick Fish

Premium Tropic Marin Pro Reef Sea Salt from Germany

Premium Tropic Marin Pro Reef Sea Salt from Germany

There is simply NO BETTER Reef Sea Salt (marine fish too)

*UV Sterilization, Sterilizer Use
The above article is the most in depth and constantly researched/updated article to be found ANYWHERE on the Internet!

TMC Total Dissolved Solids Monitor

An excellent stand alone product for testing your TDS from Tap, well, or even aquarium water.
A must have for any advanced aquarium keeper.

*Ich; Lifecycle, Identification, Treatment, Prevention

Authorized Seller of Aquaray Lighting

AAP AquaRay Ultra Premium Aquarium LED Lights
Highest in PUR, The ONLY LED with an IP67 rating or higher for water proofing along with a full 5 year warranty to back them up!
Why purchase brands without this rating such as the Finnex, Current, or Fluval only to be essentially placing an electronic light emitting device over your humid aquarium with little or no guarantee? In the long term, you WILL PAT MORE!

*Pond Care Information

*Aquarium Forum; Everything Aquatic & Aquarium Forum Board

*Fish as Pets Aquarium NewsFISH AS PETS
Fish as Pets contains articles and commentary of Interest to the Aquarium Hobby

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In Chronological order of writing with the newest at the top

  1. How to Treat Sick Fish
  2. Whirling Disease in Fish
  3. Reef Aquarium Chemistry Maintenance
  4. Use of RO, DI, Softwater in Aquariums
  5. Lighting Theory of a Planted Aquarium- RQE, PFY, PAS, & PUR
  6. Aquarium or Pond Bio Load
  7. Tuberculosis in Fish
  8. PUR vs PAR in Aquarium Lighting
  9. Head Pressure in Aquarium and Pond Water Pumps
  10. Fin/Tail Rot For Betta & ALL Fish
  11. Angelfish Virus/Aids
  12. Activated Carbon
  13. Fish Baths/Dips as an aid to treatment
  14. Streptococcus gram positive bacterium in aquariums, Eye Infections
  15. Hydrogen Sulfide
    production in anaerobic De-Nitrification for Aquarium/Ponds
  16. Fish Shipping
  17. Aquarium Size, Fish Stunting
  18. Aquarium Algae,
    BBA & Brown Algae in particular
  19. Aquarium Salt (Sodium chloride) in Freshwater Aquariums
  20. Betta Habitat; Wild Bettas to Domestic Betta environment parameters
  21. HITH; Hole in the Head Disease
  22. Aquarium Protein Skimmers, Ozonizers
  23. Power Head/ Water Pump Review
  24. Molly Disease/ Mollies in an Aquarium
  25. Basic Fish Anatomy, Fin Identification
  26. Aquarium Moving/ Power Failures
  27. Octopus as Aquarium Pets
  28. Aquarium Nitrates
  29. Ichthyophonus protists, fungus in fish
  30. Aquarium and Pond Filter Media
    Types; Mechanical, Bio, Chemical
  31. Aquarium Water Conditioners (also Pond)
  32. Fish Parasites; Trematodes & Monogeneans; Annelids and Nematodes;
    Flukes, internal worms, Detritus Worms (often confused with Planaria), Micro Worms
  33. Aquarium Silicone Application;
    DIY Aquarium Repair & Glass thickness
  34. Pond Veggie Filters; DIY Bog Filter
  35. The difference between Plaster of Paris and Aquarium Products such a Wonder Shells:
    Identification, prevention & Treatment
  37. AQUARIUM TEST KITS; Use & Importance
  38. SEXING FISH; Basics
  39. Chocolate Chip, Knobby and Fromia Starfish
  40. Freshwater Velvet & Costia
  41. Usnic Acid as a Fish Remedy
  42. Aquarium Heaters; Types, information
  43. The Lateral Line in Fish, Lateral Line Disease
    or Head and Lateral Line Erosion (HLLE)
  44. Tap Water use in Aquarium; Chloramines, Chlorine
  45. Can Black Ghost Knife fish give an electric shock?
  46. Bio Wheel Review; Do Bio-Wheels really work?
  47. How do Fish Drink?
    Use of RO Water
  48. Cyclops, and Predatory Damselfly larvae
  49. Betta with Dropsy;
    Treatment and Prevention of DROPSY in all fish
  50. pH and KH problems in African Cichlid Aquarium
  51. Aquarium Gravel, which size?
  52. Blue green algae, Cyanobacteria in Ponds/Aquariums

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UV Replacement Bulbs/Lamps Directory:
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Fish Food Information:

Aquarium & Pond UV Sterilizer Use Articles
-Unique articles such as unique UV ideas as well as dispelling myths

Directions/ Instructions/Downloads; Aquarium Pond Products

Aqua UV versus TMC UV Sterilizer
VIDEO: Aqua Pond UV vs TMC AAP Pond UV Clarifier Sterilizer

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