Pond salt, sodium chloride, NaCl, has been termed "The KOI Wonder Drug". A misnomer perhaps, but salt is a proven staple in the health care and maintenance of Koi worldwide. Koi maintain an internal concentration of salt in their body fluids higher than that of their liquid environment. The addition of one to two pounds of salt per hundred gallons of pond water (0.1% to 0.2%) is recommended for most ponds, especially in the Spring and Fall. This is a fairly conservative dosage but unless one has an accurate measurement method, higher concentrations should be avoided. If nitrite is present, two pounds of salt per hundred gallons is appropriate to reduce nitrite toxicity.
The amount of pond salt dissolved in water is termed the salinity and is measured either as a per cent (%), in parts-per-thousand (ppt), or in parts-per-million (ppm) (where 10 ppt = 1% = 10000 ppm). The more common parts-per-thousand measurement is the weight of the salt in pounds per thousand pounds of water (about 125 gallons). Since 100 gallons of pure water weighs about 800 pounds, one pound of salt per hundred gallons equates to a salinity of 1.25 ppt (0.125% or 1250 ppm). (1 ppt = 0.8 pounds per hundred gallons).
Applying Pond Salt
Introduce the salt, if possible, at the discharge side of the waterfall box or in the stream bed. If the addition must be
made directly into the pond, dissolve the salt in a bucket of pond water and distribute it evenly around the edges of the pond. Inquisitive Koi will check to see if any new addition to the pond might be something to eat. Although they will probably not swallow the pieces of salt, direct contact of crystalline salt with the fish for more than a few seconds can cause injuries similar to burns.