If you are going to use salt (sodium chloride) to treat freshwater fishes, you use 1 heaped tablespoon of rock salt, sea salt or swimming pool salt per 20 litres (5 gallons) of water. If there is no improvement after 48 hours you can double that dose so there is 2 heaped tablespoons of salt per 20 litres of water.
This dose rate is for soft water fish like tetras, barbs, gouramis, Corydoras, angelfish, rasboras & loaches.
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If you have livebearers, rainbowfishes or other fish that can tolerate salt, then you start with 2 heaped tablespoons per 20 litres and if there is no improvement after 48 hours, you can double that so there is 4 heaped tablespoons of salt per 20 litres.
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You add the salt to the tank and keep the salt in there for at least 2 weeks but no more than 4 weeks otherwise kidney damage can occur. Kidney damage is more likely to occur in soft water fishes than in fish that can tolerate salt. The fish live in the brackish (salty) water and you do not move them out or give them dips.
After the fish have recovered (2-4 weeks), you do a 10% water change each day for a week. Then 20% each day for a week. Then you can do bigger water changes again.
The salt will not affect filter bacteria, however the higher dose rate (4 heaped tablespoons per 20 litres) will affect some plants.