A substance when dissolved in water has a natural desire to dilute itself by bonding with surrounding water molecules. When two solutions are separated by a semipermeable membrane, such as a cell membrane, pure water flows from the weaker solution to dilute the stronger one until they are both the same strength. The term used to describe the flow of water through a semipermeable membrane separating two solutions of different concentrations is osmosis.

Osmosis will stop when the two solutions reach equal strength, and can also be stopped by applying a pressure to the liquid on the stronger-solution side of the membrane. The pressure required to stop the flow from a pure solvent into a solution is a characteristic of the solution, and is called the osmotic pressure.

Osmosis is important to fish as their body fluids are more concentrated that the surrounding water. Water moves by osmosis through the semi-permeable membrane of the fishes skin and gills and constantly dilutes their body fluids. Freshwater fish must excrete wastes to prevent themselves becoming waterlogged. Marine fish have the reverse problem in that sea water has a higher concentration of dissolved salts than the body fluids of the fish. They must therefore drink plentifully to avoid drying out. Fish control the intake of and expulsion of fluids by osmoregulation.

The concentration of dissolved salts, which accounts partly for the difference in concentration of a solution, can be interpreted in terms of specific gravity or "salinity". Specific gravity can be measured using a hydrometer.

Fish are in osmotic balance with their natural environment but are often unable to cope with water of a different salinity because they cannot control the influx of water into their body. Too rapid a change in salinity (and hence osmotic pressure) can cause osmotic shock to most fish. This is most critical when fish are breeding as single cells, such as eggs and sperm, are very sensitive to osmotic pressure. For example, if a fish adapted to soft water deposited its eggs in hard water, the eggs would shrivel up as they loose water to the high salinity of the water around them.

In freshwater aquariums, the addition of common salt to the water (increasing the specific gravity) can reduce the osmotic pressure between the water and the body fluids of the fish. While some fish are happy in slightly salty water naturally, such as brackish water fish, the addition of a small amount of salt when fish are ill reduces the osmotic pressure on them i.e. they do not need to use as much energy to excrete large quantities of water from their bodies due to osmosis. As a general guide to adding salt as an aquarium tonic, a 0.1 percent solution is recommended (1 teaspoon per imperial gallon).