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.

Alkalinity Carbonate hardness in Ponds
Alkalinity, often referred to as -carbonate hardness, or German carbonate hardness, is the measure of carbonate and bicarbonate concentrations in your aquarium water. Alkalinity is a measure of the...(click on product image for more details)
Ammonia in Ponds
Where does the ammonia originate from Pond fish produce waste in the form of ammonia, which is released into the water through the gills. Ammonia can also originate from the dead and decaying plant...(click on product image for more details)
Chlorine in Ponds
Acceptable concentration 0 A gas widely used in the disinfection of water and as an oxidizing agent for organic matter, manganese, iron, and hydrogen sulfide. Chlorine is known to react with...(click on product image for more details)
Green Water Algae in Ponds
Sometimes referred to as an algae bloom, Another factor which can influence the pH of the water is the presence of plants, most notably if there is an algae bloom in the pond. During the hours of...(click on product image for more details)
Nitrate in Ponds
Nitrate, NO3-N, Nitrate is the final product from the breakdown of ammonia released by the fish. Nitrate is not especially harmful to freshwater fish but is a potent plant fertilizer and can...(click on product image for more details)
Nitrite in Ponds
As the ammonia in the water begins to reduce, the secondary break down product, nitrite will begin to increase and this is also very poisonous to fish. Nitrite is a skin irritant and will cause the...(click on product image for more details)