In some cases you can treat fin rot with salt but this remedy should only be used on salt-tolerant freshwater fish. You can use tonic salt, or sodium chloride, adding 1 ounce (28g) of salt per gallon (3.7 liters) of tank water. Some fish owners advise using it only when the fin rot is particularly aggressive. You can also add aquarium salt to the water after cleaning the tank (check it’s suitable for freshwater fish), so the water is healthy––this can be done after each cleaning.