Common salt, Sodium Chloride (NaCl) is commonly used in the health care and maintenance of fishes especially Koi.  Salt is useful in treating parasites.  It will eliminate seven out of nine parasites that are commonly found in Koi pond.

Most fish have an internal salt concentration of 1.0% or 10 parts-per-thousand (ppt) which is higher than their water environment. 

Osmosis causes water to transfer from the lower salinity of the pond water into the tissues of the fish.  Adding salt will lower the osmosis pressure.  This reduces the effort of the fish in eliminating the excess water.  The saved energy is then available for use by the fish’s own immune system to fight diseases.   However, if the salinity of the water environment is close to the internal tissues of the fish, the osmosis pressure will reverse and cause the fish to die of dehydration.

Salt concentrations between 0.3% and 0.5% (3 to 5ppt) will upset the osmotic balance of some parasites i.e. the parasites’ cells will dehydrate.  Salt concentration of 0.3% are effective at detoxifying nitrite and concentration of 0.25% or higher has been used to control string algae. 

Be aware that, salt can cause pond plant damage as the concentration increases.   Floating plants (water hyacinth, water lettuce, etc.) are affected at lower concentrations than most bog plants.