PROPER OSMOTIC FUNCTION- ELECTROLYTES; REVERSE OSMOSIS & SOFT WATER USE IN AQUARIUM, HOW DO FISH DRINK
This Includes These Sections:
*How do fish drink water

*The Importance of healthy Osmoregulation/ Osmotic Function in Fish, this includes Osmotic Function in Freshwater, Saltwater, & Anadromous/ Catadromous/ Amphidromous Fish

*Reverse Osmosis Filter Information, Use;
Including Information about the Correct Use of RO, DI (or Distilled), or Rain water in Aquariums or Betta Tanks

*Why to NOT use Water Softener Water (or sodium based products) in Aquariums

* Necessary Minerals

Updated 12/01/12

QUESTION: How do fish drink water?

ANSWER:
Freshwater fish absorb most of the water they need through their skin via osmosis by producing dilute urine and actively transport essential mineral ions from the surrounding water to compensate for mineral ions lost via the urine and diffusion from the gills (osmosis is the net movement of water through a selective permeable membrane from a region of low solute potential to a region of high solute potential due to their hyperosmotic environment), NOT through their gills. The gills are for respiration.

Most saltwater fish (Hagfish, Sharks, Rays differ in osmoregulation) actually drink the water the live in, as the salt in the water is constantly pulling H2O from their bodies in a reverse respiration as their tissues are hypotonic compared to surrounding water and they must ingest large volumes of water and actively excrete mineral ions (hypo-osmotic environment).
Marine Bony fish (not Rays, Sharks Hagfish) maintain their osmotic concentration at about one quarter to one third that of sea water. Normally in salt/sea water, bony marine fish have a tendency to lose water from their gills due to osmosis as well through their urine. Marine fish have to drink a lot of water to make up for the loss, however, as the water contains a lot of salt (35% or approximately 1.025 specific gravity) they must remove the excess salt from their system. The sodium and chloride ions are secreted by the gills and magnesium and sulphates are excreted in urine. An important side note is this is an active process and requires much energy which is why lowering salt levels (NOT other mineral ions) can be helpful in aiding a sick bony marine fish! It is noteworthy that Hagfish, Sharks, Rays have similar concentrations of salts to that of marine bony fish, however, they also have very high concentrations of organic compounds which gives their internal fluids the same osmotic concentration as sea water so these fish due not generally benefit from lower salt levels when sick.

These different abilities explain why some fish such as catfish are sensitive to salt in the water, but this is also why some fresh water fish are helped by salt to generate a mucous slime coat on their skin which is necessary for disease prevention.

For proper osmoregulation electrolytes such as calcium, magnesium and other elements are important as well.
This is important and not realized by many aquarists (especially in freshwater), however not having these electrolytes (minerals) present in the water whether by depletion or by the use of drinking water, distilled water or RO water that has not been re-mineralized can cause problems with the fish’ ability to move fluids in and out of their bodies and in the long term resist disease.

Another note, because most freshwater fish cannot drink their surrounding water (Salmon and others are exceptions), when you place these freshwater fish in saltwater, they actually dehydrate.

Osmosis in fish;

Their cells must always be bathed in a solution having the same osmotic strength as their cytoplasm. This is one of the reasons why fish and other animals have kidneys. The exact amount of water and salt removed from their blood by the fish kidneys. The process of regulating the amounts of water and mineral salts in the blood is called osmoregulation. Fish which live in the sea (remember the sea is full of salt and other elements), but fish which live in freshwater have the opposite problem; they must get rid of excess water as fast as it gets into their bodies by osmosis. Osmosis is an important topic in biology because it provides the primary means by which water is transported into and out of cells.

Osmosis is also important in the treatment of many aquarium diseases in both freshwater and saltwater. A general breakdown in osmoregulation due to disease, poor water quality (especially the lack of essential mineral electrolytes such as Calcium, magnesium, and Sodium) is often responsible for the bloated condition that results from excess water accumulation in tissues. It is common for Fish in this condition often rapidly succumb due to loss of homeostasis (the constant internal environment), essential to carrying out metabolism and other life activities this tends to be more common among FW fish in my experience, in part due to the lack of understanding of the role that many essential minerals play in essential life processes of fish.

Generally salts (trace elements), not just sodium chloride can affect osmosis. Magnesium can also play a major role too. Calcium can affect and just as importantly be affected by proper osmotic function.
Sulfates have been shown effective in improving nutrient absorption and toxin elimination. Magnesium plays a role in the activity of more than 325 enzymes and aids in the proper assimilation of Calcium.
Often many aquarists understand how salt (sodium chloride) affects osmoregulation and the popular question of “Do fish drink?” however this is a dangerous over simplification as although sodium chloride (as often represented as sodium and chloride) are important, the lack of other essential elements including Calcium (which the absence Calcium of in fish can lose/leak substantial quantities of other minerals/salts into the water).
Here is an over simplification I read recently that is not necessarily wrong, but in misleading in that it implies salt is the only essential mineral: “They (freshwater fish) absorb water through their skin and have effective ways of excreting excess liquid to maintain the salt they need”. The implication is that the fish basically just needs to maintain salt and/or this or other minerals somehow take care of themselves. The facts are that without Calcium (as tested via GH), the fish CANNOT properly osmoregulate.

For much more information about the importance of Calcium and other electrolytes, please read this article (in particular the section about Calcium):
CALCIUM, KH, AND MAGNESIUM IN AQUARIUMS; How to maintain a Proper KH, why calcium and electrolytes are important.

FRESHWATER FISH

In freshwater, a higher electrolyte level (particularly of sodium chloride, calcium and magnesium) will help pull fluids through the body which also stimulates the natural mucous coat on fish so as to resist parasites, bacteria, and fungus. Also by pulling fluids through the body this can help with bloat, swim bladder problems, intestinal problems, and even dropsy (which I have had few problems with in clean tanks with good electrolyte/ trace element levels).

This process results in the loss of many electrolytes, some of these trace elements can be replaced by ions contained in food but by far the most common method is through the movement of a substance against an osmotic gradient through the use of energy. This usually involves the exchange of one substance for another. In the case of freshwater fish, Na+ (sodium) ions are taken from the water and ammonia ions are taken from the fish and they are exchanged. This effectively rids the fish of ammonia. Chloride ions are exchanged for carbonate ions which help in maintaining the pH of the body fluids.
This is one more important reason for adequate Calcium, carbonate (KH), & electrolyte levels
Opportunistic diseases such as Columnaris, Saprolegnia (often known as fish fungus), and Aeromonas (often the cause of Septicemia) are more easily prevented when osmoregulation is functioning properly in fish via adequate mineral levels.

However it should also be noted that before you go an dump a lot of sodium chloride (salt) in your freshwater aquarium, even for fish such as African Cichlids where this is a common practice that overuse of salt can have negative side effects such as loss of other essential elements/minerals and general osmoregulation (& is occasionally noted as a contributing factor for the condition of Malawi Bloat). Put another way, you want to achieve and Osmotic balance. I prefer to use salt in fish such as African Cichlids sparingly and then bring it “up” during times of stress or suspected disease, only to bring it to low levels such as just a 1 teaspoon per 5 gallons reserving higher salt concentrations for 30 minute “salt baths” of 1-2 teaspoons per gallon when necessary for treatment/therapy. What I (and many good research papers) find is that maintaining healthy levels of Calcium, magnesium, sodium, and Carbonates is far more important for long term osmoregulation and fish health, whether it be African Cichlids or Discus (the LACK of these other elements are often a common factor in cases of Malawi Bloat).

For much more about the correct use of salt (NaCl) in freshwater aquariums, please see this article:
“Aquarium Answers; Aquarium Salt (Sodium chloride) in Freshwater Aquariums

SALTWATER FISH

A saltwater (marine) fish in water with a slightly higher salt level will devote more molecular energy to osmoregulation; following this logic, if a fish is suffering from stress lowering the salt level will help the fish to recover. Lowering the salt level (salinity) will reduce osmotic pressure within the fish allowing it to allocate more of its molecular energy to its immune system, this will help stop the stressed fish from becoming sick and can be used to treat fish that are sick.

As well, in marine fish sometimes lowering salinity will have a reverse osmotic effect and rupture the cell wall of many parasites such as Oodinium and Cryptocaryon (this is best achieved in a 3-5 minute freshwater bath adjusted for pH).
This method of lowering the specific gravity (salt content) in saltwater to fight disease should not be taken too far. I have heard of persons being told to keep their marine aquariums at a specific gravity of 1.012 to prevent of fight disease, however this is TOO LOW. At his specific gravity (salinity), the marine fish will not have proper osmotic function (remember, marine fish drink the water around them and at this salt level they will not get the fluids and minerals being pulled properly through their bodies which can result in water retention and MUCH worse). The general specific gravity in marine aquariums should be around 1.019 to 1.022 for fish and around 1.022 to 1.025 for reef.
Be careful when lowering salinity as marine fish can generally only handle about 2 degrees of salt change per day (example; 1.025 to 1.023).
To treat parasite infections (such as Oodinium and Cryptocaryon), you can TEMPORARILY and slowly lower the specific gravity to as low as 1.010.
Be careful in lowering salinity with corals and anemones present as they cannot tolerate the lower salinity levels fish can.

ANADROMOUS, AMPHIDROMOUS, & CATADROMOUS FISH

These are fish that can migrate between fresh and saltwater.
• Anadromous fish live in the sea mostly, and then breed in fresh water such as the Pacific Salmon
• Catadromous fish live in fresh water and then breed in the sea such as freshwater eels of genus Anguilla
• Amphidromous fish move between fresh and salt water during some part of life cycle, although not for reasons of breeding such as Bull sharks in the Zambezi river of Africa and living in Lake Nicaragua of Central America.

Using Salmon as an example of how bony fish (other than sharks, rays, true eels) osmoregulate in these dual conditions;

Salmon spend most of their life in the open ocean, where they reach sexual maturity, but lay their eggs gravel beds at the upper reaches of freshwater streams. When the eggs hatch, the young salmon spend several months migrating downstream to the ocean where they remain for some 3-5 years. When mature, the adult salmon return to mouth of stream where they hatched migrating upstream to its headwaters, spawn, and die.
There are osmoregulation/ physiological challenges presented by habitats as different as freshwater streams and the open ocean for which the salmon must adapt to so as to complete this cycle. The salmon uses adaptations, both behavioral and physiological, that allow it to thrive in both fresh and salt water habitats. To offset the dehydrating effects of salt water, the salmon drinks copious amounts of saltwater so as an ocean-dwelling salmon drinks, it takes in a lot of NaCl, which exacerbates the salt-loading problem. However in fresh water the salmon doesn't drink at all.

Kidney function also differs between the two habitats. In fresh water, the salmon's kidneys produce large volumes of dilute urine (to cope with all of the water that's diffusing into the salmon's body fluids), while in the ocean environment, the kidneys' urine production rates drop dramatically and the urine is as concentrated as the kidneys can make it. The result of this is that the salmon is using relatively little water to get rid of all of the excess ions it can.

Finally, the other adaptation salmon use to deal with the NaCl fluxes driven by the gradients between the salmon and its surroundings. In their gill epithelial cells, salmon have a special enzyme that hydrolyzes ATP (Adenosine triphosphate (ATP) is the universal unit of energy used in all living cells ) and uses the released energy to actively transport both Na+ and Cl- against their concentration gradients. In the ocean, these Na+-Cl- adenosine triphosphatase molecules 'pump' Na+ and Cl- out of the salmon's blood into the salt water flowing over the gills, thereby causing NaCl to be lost to the water and offsetting the continuous influx of NaCl. In fresh water, these same Na+-Cl- adenosine triphosphatase molecules 'pump' Na+ and Cl- out of the water flowing over the gills and into the salmon's blood, thereby offsetting the continuous diffusion-driven loss of NaCl that the salmon is subject to in fresh water habitats with their extremely low NaCl concentrations.

When a young salmon on its seaward journey first reaches the saline water at the mouth of its home stream, it remains there for a period of several days to weeks, gradually moving into saltier water as it acclimates. During this time, it begins drinking the water it's swimming in, its kidneys start producing a concentrated, low-volume urine, and the NaCl pumps in its gills literally reverse the direction that they move NaCl (so that they're now pumping NaCl out of the blood and into the surrounding water.
Likewise, when an adult salmon is ready to spawn and reaches the mouth of its home stream, it once again remains in the brackish water zone of the stream's mouth until it is able to reverse the changes it made as a juvenile invading the ocean for the first time.

Further reference:
http://www.unm.edu/~toolson/salmon_osmoregulation.html

RO (REVERSE OSMOSIS), DI (DI-IONIZED or DISTILLED), RAINWATER IN AQUARIUMS, OR WATER SOFTENER WATER

*First what is RO water? 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.
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).
Most aquarium RO units use TFC (thin film composite) membranes (the other type is the CTA, a.k.a. the cellulose triacetate membrane). 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).

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

See this pdf download for further directions as to maintenance of a RO unit:
TMC Reverse Osmosis Instructions

*DI Unit; 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.

For my purposes I found that a RO only unit was/is more than adequate, however if you are making water for extremely sensitive purposes (such as a car battery), 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 removes is mineral cations (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 and in fact these 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.
The bottom line is to not believe the hype some sellers of Four Stage RO/DI units put forth and save your money (both initially and ongoing) and purchase a RO only Water systems.

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

Use of RO, DI (Distilled) water in Aquariums or Betta Tanks;
Unless you are an advanced fish keeper (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% to 50% 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).

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 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.

For those considering using RO or DI water in higher amounts, it is important to note that for proper osmotic function trace amounts (and I mean TRACE amounts) of several minerals are required, many 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 Wonder Shells , SeaChem Replenish 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).

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):


*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.

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:

*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.
*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 Additives:

*SeaChem Alkaline Buffer along with SeaChem Acid Buffer 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 Shells (an important compliment mineral replenisher; mixes well with Electro Right, Replenish, RO Right, SeaChem Buffer) or the similar SeaChem Equalibrium. An excellent compliment to follow up with Replenish or other intial RO Water Electrolyte additives, so as to constantly maintain essential mineral cations.
*SeaChem Equalibrium; Similar to Wonder Shells or AragoMight, but with more emphasis on Potassium and thus planted aquariums. In most cases I have found this product NOT the better choice.

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 (which 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 has 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.

Softened Water; Home/Office Water Softeners Use:

Home (or office) water softeners that employ salt 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), this can severely affect osmoregulation in fish, especially many fish such as Loaches that normally prefer more soft water, but not soft water containing an unbalanced mineral content that is high in sodium but missing other essential mineral ions. The use can cause stress in fish, from low to severe.

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.

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

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

Necessary Minerals

Here are a few necessary TRACE elements/minerals (electrolytes) and their function. Please read the above article for more about Calcium.

• Calcium (Actually needed in more than trace amounts): Calcium helps to transport ions (electrically charged particles) across the membrane, is essential for muscle contraction, calcium assists in maintaining all cells and connective tissues in the body, and much more
• Sodium (Actually needed in more than trace amounts which is why water from home water softeners should NOT be used): Regulates extra-cellular electrolyte, essential for the transport of nutrients across the cell membranes.
• Potassium: Regulates intracellular osmotic pressure, cell membrane potential, and salt excretion.
• Phosphorus: Energy metabolism.
• Molybdenum: Important for proper skeletal growth (very important in reef aquariums for hard coral growth).
• Manganese: Aids enzymes involved in metabolism, growth and maintenance of bone and cartilage.
• Iron: Oxygen transport in blood and muscle tissue.
• Magnesium: As stated previously, magnesium plays a role in the activity of more than 325 enzymes and aids in the proper assimilation of Calcium.
• Sulfates: Also as stated above, improve nutrient absorption and toxin elimination.
• Chromium: Important for proper utilization of sugars.
• Cobalt: Necessary for Folic Acid synthesis.
• Copper (very trace amounts): Co-enzyme for energy metabolism, aids in the protection of the myelin sheath around the nerves, important for iron absorption and utilization.

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 Redox Potential of approximately -300 mV via proper dissolved oxygen levels, calcium and other electrolytes, proper cleaning procedures and water changes (UV Sterilization can help too).
Redox is NOT a water parameter the average aquarist needs to worry about, as good aquatic husbandry usually takes care of this however it is still a useful parameter to be familiar with.
For more information about the Redox Potential, please see my article: The Redox Potential in Aquariums (& Ponds) and how it relates to proper aquatic health

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

This one is great Understanding Water Hardness

A little hard to follow (pdf), but good reading:
Interactions of pH, Carbon Dioxide, Alkalinity and Hardness in Fish Ponds

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

This article is a very interesting (although somewhat dated), especially as it pertains to the benefits of lower salinity for Marine teleost/teleostei fish (ray-finned fishes):
Osmoregulation (http://www2.hawaii.edu/~delbeek/delb11.html)
This point from this article is noteworthy:
"Since marine fish must constantly expel various solutes, such as sodium and chloride ions, against an osmotic gradient, a great deal of energy is required. Therefore, anything that you can do to lower the osmotic gradient will benefit the fish in terms of energy expenditure. The simplest way of doing this is to lower the salinity of the water as much as possible, particularly for a fish in distress."

For more aquarium information, please visit this site:
“AQUARIUM AND POND INFORMATION”

For a VERY knowledgeable aquatics forum with a lot of aquatic experience: Everything Aquatic

Other Resources:
Sodium Ions: http://www.elmhurst.edu/~chm/vchembook/142Aposion.html

Also see this article for the Internet's Best Aquarium Lighting Information: Aquarium Lighting;Facts & Information

Copyright Carl Strohmeyer 12/04/06

Other Recommended Reference Sites

-A useful source for current Aquarium Information and Resources (Pond too). Basic and in depth articles from Aquarium Lighting; Complete Information, Filtration, Fish Nutrition, UV Sterilization; Aquarium or Pond, Ich, Pond Care, Nitrogen Cycle, and much more. Well researched and up to date aquarium and pond articles, answers, help, and links. Based on 33 years Professional experience & research in Los Angeles and now in Oregon. This Aquarium and Pond Information resource is a must read for any aquarist serious about current aquatic information and articles

For a friendly, Knowledgeable, aquarium forum with in a family atmosphere, Aquarium Forum; Everything Aquatic & Board is an excellent place to go for information, help or simply to share your love of the aquarium and pond hobby and help others. A superior place for information over such places as Yahoo Answers

FISH AS PETS; Articles and commentary of Interest to the Aquarium Hobby; Such as Parasite Retailers, Planaria & Detritus Worms in Aquarium, Melafix Dangers, & Celestial Pearl Danio, Galaxy Rasboras

Labels: Amphidromous, Anadromous, aquarium DI water, aquarium RO water, aquarium soft water, Catadromous, Do fish drink, osmoregulation, reverse osmosis, RO fish