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Proper Osmotic Function in Aquarium Fish - by Carl Strohmeyer

QUESTION: How do fish drink water?

ANSWER: Freshwater fish absorb most of the water they need through their skin via 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), not through their gills. The gills are for respiration.
Saltwater fish actually drink the water the live in, as the salt in the water is constantly pulling H2O from their bodies in a reverse respiration, 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. Proper electrolytes, magnesium and other elements are important for slime coat generation.

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.

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.

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.

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 or Distilled water are not good for water changes unless re-mineralized or blended with tap or well water that is possible 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” 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.

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 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 my FULL article about proper osmotic function in fish including a list of necessary trace elements for fish, please visit this link: http://aquarium-answers.blogspot.com/2006/12/how-do-fish-drink.html

Professional aquarium maintenance experience since 1978 as the owner of one of the larger aquarium maintenance companies in LA, CA.

I have been in the hobby since 1969.



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