Osmosis in the Aquarium
Ardan Huck June 23, 2002

Osmosis is very important to fish and if we understand the principles involved, we can use it to our advantage in helping our fish in times of stress.

Osmosis is the process where water moves through a semi-permeable membrane (fish skin or gill membrane) from a lower concentration of minerals (aquarium water) to a higher concentration of minerals (fish's body). The semi-permeable membrane (fish skin or gill membrane) only allows water to pass through it, in and out, not the minerals. This is how freshwater fish stay hydrated. Saltwater fish have to drink lots of water and retain water through their kidneys, excreting minerals, to stay hydrated.

"Osmotic Pressure" is the "force" on the water to move it from the low concentration of minerals to the high concentration of minerals. As water enters the cell, the cell membrane expands and causes "osmotic pressure" to rise within the cell. This helps control the amount of water entering the cell. Freshwater fish excrete water as "dilute urine" from the kidneys. This keeps them from becoming "waterlogged" (Dr. Carrington 1985).

If we add salt to the aquarium water, increasing the mineral concentration outside the fish, the process is reversed and some of the water is "drawn out" of the fish's cells, or it limits the amount of water that enters the fish's cells, decreasing the pressure within the fish cells, making the fish "feel better." They also do not have to use the kidneys as heavily to excrete excess water.

If the concentration of the salt in the water is high, as in a "salt bath," water is drawn out of cells, such as bacteria or parasites, and destroys them. The concentration and length of time in the bath and the type of cell walls determine which cells survive and which don't.

Osmosis is also affected by the depth of the water that the fish is in. This is called "hydrostatic pressure" (water pressure). Therefore, the deeper the fish goes, the more water pressure outside the fish, resulting in "more" water wanting to flow "into" the fish's cells.

"Reverse Osmosis" is the process of water moving through a semi-permeable membrane (only allowing water to pass through) from a high concentration of minerals to a low concentration of minerals through the use of "water pressure" (hydrostatic pressure).

  • "Maintaining a Healthy Aquarium" by Dr. Neville Carrington, Salamander Books 1985
  • "World Book Dictionary" by Clarence L. Barnhart and Robert K. Barnhart, World Book Inc. 1991
  • "World Book Encyclopedia" by World Book Inc. 1990
  • "Anatomy and Physiology" by Anthony and Thibodeau, C.V. Mosby Company 1983

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