Salt in a freshwater aquarium is not as strange as it may seem at first, freshwater fish have the same salt levels in their blood as their marine counter parts, about 9ppt which was the same amount as the ancient seas that all fish evolved from. 

These body salts (mostly sodium chloride) are lost through osmosis when water passes over the gills and must be kept in balanced or the fish risk “drowning from within”. This balancing act is called Osmoregulation and is achieved by the fish drinking very little, having super efficient kidneys which produce large amounts of dilute urine and possessing special chloride cells that pump salt into the body.

This osmoregulation can use up to 30% of a fish’ energy and if it has a wound or ulcer it has to step up this osmoregulation. This can be fatal especially if there already underlying issues such as kidney disease. Stress also affects this process as it changes many hormonal and physiological things, such as an increase in the uptake of oxygen through the gills which allows more water into the fish, creating serious imbalances in its osmoregulation.

Salt can also provide other benefits such as killing certain pathogens like fungi, skin flukes and Ich parasites. But only ones that live on the surface, internal ones are not affected. Salt treatments vary from pathogen to pathogen so get in touch if you’d like some recommendations. Aquarium, or tonic Salt is often used but it is a medication so it should not be in your regular routine unless you are treating something specific. Once treatment is complete, perform 50% water changes every few days until you have less than 1g per Litre.