Effects of Low Salinity Levels on Fish

Water will always flow from a high concentration area to a low concentration one in order to find balance. In ocean conditions, fish drink a significant amount of water as they are constantly losing it to the lower concentration of the surrounding saltwater. They also must pump out the excess salt through their kidneys.

Salinity refers to the salt content of water. It is often measured in terms of specific gravity, or the amount of dissolved salt. In general, fish thrive in an intermediate specific gravity of 1.023 to 1.025. According to a study on Atlantic cod, hypersalinity -- too much salt -- is more damaging than hyposalinity on fish growth. However, there are short-term benefits to reducing specific gravity (i.e., 1.001 to 1.002). An aquarist can provide measurement tools to monitor tank salinity.

Like all organisms, fish are susceptible to parasitic infections like marine ich. This primarily presents itself with characteristic white spots on the fish but also involves breathing problems, lethargy and abnormalities to the fins, eyes and scales. Low salinity has been shown to be an effective treatment. Reducing the specific gravity to 1.009 to 1.010 at a temperature of 78 to 80 degrees Fahrenheit for two weeks eradicates ich by allowing the fish to heal without causing organ failure and death.

While hyposalinity has short-term benefits for combating parasites, a low salt environment over the long term can be a stressor for most fish. To maintain their internal equilibrium, fish drink water and flush the excess salt out of their systems, continually repeating this process. But if the surrounding water has a low specific gravity, fish kidneys may shut down. It's always best to consult an aquarist before utilizing a hyposalinity treatment as certain organisms cannot tolerate it, even in the short term.