Sodium chloride (salt; NaCl) has been used for freshwater fish to decrease stress and manage a variety of disease conditions. Salt works well against many protozoan parasites such as Costia, Trichodina and Chilodonella as well as flukes and other ectoparasites. It can also assist osmoregulation problems caused by bacterial ulcers; help clear congested gills as well as supporting fish suffering from stress.
The method of salt application depends on the disease organism, fish species, weight, and type of aquaculture unit. Treatment methods include short-term dips, prolonged baths, and indefinite treatments.
Up until the 1980s, most fishkeepers believed that water changes should be minimised. 'Old water' was thought to be good for the fish, so water changes were often limited to around 20% per month. With such small water changes, nitrate levels were often very high. At high concentrations, nitrate can be toxic to fish, but sodium chloride reduces the toxicity of nitrate. While aquarists didn't necessarily know this, what was observed was that fish were often healthier in tanks with a bit of salt added to the water.
Salt reduces the toxicity of nitrite and nitrate. While nitrate rarely reaches dangerous levels in a properly maintained aquarium, nitrite levels above zero are commonly seen in overcrowded or immature aquaria. As a short term measure, the addition of 1 gramme of salt per litre of water will reduce stress and minimise the problems caused by stress, such as finrot. Note that the use of salt doesn't eliminate the need to fix whatever problems are causing the non-zero nitrite level.
Salt is an extremely effective curative in the freshwater aquarium to prevent and cure diseases. It is known to destroy many kinds of bacteria, to increase slime-coat production , and even to cause some parasites to drop off. Salt is one of the most widely used medications in freshwater tropical fishkeeping, but that doesn’t mean it’s used properly. While it can be very effective in certain situations, in the wrong situations salt can do more harm than good.
DISCLAIMER: Freshwater aquarium salt is not recommended for some planted aquariums or aquariums that house some scaleless fish, such as Corydoras, because certain plants and fish are extremely sensitive to salt. Freshwater aquarium salt is also not a substitute for marine salt because they are designed for very different purposes.