PROPER OSMOTIC FUNCTION- ELECTROLYTES

QUESTION: How do fish drink water?

ANSWER: Freshwater fish absorb most of the water they need through their skin via osmosis (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 (hypoosmotic environment). This is 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 often 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.
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.



In saltwater, 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. To treat parasite infections (such as Oodinium and Cryptocaryon), you can TEMPORARILY lower the specific gravity to 1.016 to 1.018.
Be careful in lowering salinity with corals and anemones present as they cannot tolerate the lower salinity levels fish can


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

RO (Reverse Osmosis), DI (Distilled), or Rainwater in Aquariums

The use of RO or DI water in aquariums should be restricted to blending with tap or well water to have a lower naturally buffer aquarium for Amazon River or similar fish. Generally I start with 25% and work up from this over time. 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.
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 and buffers) or blended with tap or well water that is possibly too high in many minerals (a very high GH can slow respiration in some freshwater fish).
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.

Finally 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.

Necessary Minerals

Here are a few necessary TRACE elements (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): 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 Wonder Shells which aid in the addition of electrolytes (an excellent product I have used for years with very good results as a tool for maintaining proper water parameters): “Wonder Shells” -for calcium and electrolytes

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 article is a very interesting (although somewhat dated especially as it pertains to Wet/Dry filters, which can become “nitrate factories”):
Osmoregulation (http://www2.hawaii.edu/~delbeek/delb11.html)


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