Salt is a normal component of many fish systems. Saltwater and brackish systems require salt on a continual basis. But what about freshwater systems? Some internet pages recommend keeping salt in koi/goldfish ponds/tanks at all times. Others say don’t bother. What are the effects of salt on an all-freshwater species?
In a freshwater environment, a fish’s body is more dense than the surrounding water. As a result, water is constantly trying to enter the fish’s body and achieve osmotic homeostasis. A fish’s gills, kidneys and urinary systems are responsible for removing the excess water. When these mechanisms are compromised, the clinical manifestation of “dropsy” can occur. By adding salt to the water, you are changing the osmolality of the environment surrounding the fish, thereby decreasing the amount of water diffusing into the fish. Some people figure this is less stressful for a fish, but after millions of years of evolution, it really isn’t necessary. A fish’s body can handle the metabolic demands of freshwater. It’s the human equivalent to drinking less water so your kidneys don’t have to work as hard.
Most salt-avid fish keepers will keep their fish in very low salinities: 1-1.5%. This level can be very hard to test accurately for and will often vary between 0-3%. This level will not hurt your fish. If you feel like you NEED to have salt in there, that’s fine, but you should not strive for levels greater than 2%. You do NOT HAVE to add salt to your freshwater fish system. Always add non-iodonized salt (not table salt)!
For some parasites, depending on the species and water temperature, higher therapeutic levels of salt can be used to treat the infestation. Salt works by first increasing, then depleting the mucus coat, which many parasites use as a food source and for protection. Once the mucus coat is gone, the single-celled parasites rupture from the changing osmotic gradient. But if you are slowly bringing your salt levels up without realizing it, some parasites have been found to become resistant to therapeutic salt levels, requiring treatment at even higher doses. The higher the salt dose, the harsher the treatment for the fish.
All in all, a little salt is fine, but don’t obsess over it. Your fish will do just fine without it. The only time we recommend salt is for recovery from trauma or surgery. Don’t forget that salt can kill aquatic plants!