Common 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. Osmosis causes water to transfer from the lower salinity of the pond water into the tissues of the fish. This additional water build up must be eliminated by the kidneys. Although salt in higher concentrations may slow some disease causing bacterial growth in the pond, the predominantly accepted theories ascribe the primary benefits of salt to lowering the osmotic pressure. This reduces the effort the fish must expend in eliminating the excess water. The saved energy is then available for use by the fish's own immune system to take care of other potential problems. The presence of salt also helps counteract any nitrite toxicity. In some cold climate areas, it is added in the Winter to lower the freezing point of the water.
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. Related, salt may provide some minor control of algae in the higher concentrations.
The amount of 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). Pond-keepers often talk about the pounds of salt per hundred gallons of water. 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)
[Note: Koi internal fluid salinity is on the order of 9 ppt (about the same as ours). Sea water is around 35 ppt to 70 ppt depending upon geographical location. The Great Salt Lake has a nominal concentration of about 250 ppt.]
There is some disagreement about salt in Koi ponds. Our San Diego tap water often has a salinity of up to 0.5 ppt. This amount cannot be tasted but we drink it and we put it into our ponds as make up water. If our Koi were put into an absolutely pure (distilled) water environment, the osmotic pressure would be so high that some would be unable to eliminate the excess water and would die almost as if by drowning. On the other hand, if the salinity approaches that of the internal tissues of the fish, the osmosis process will decrease or even reverse. This can cause the fish to die, essentially of dehydration. Any discussions should therefore center not on should salt be in the pond but how much.
Salinity acceptable range: 0 - 5 ppt
The addition of one to two pounds of salt per hundred gallons of water (1.25-2.5 ppt) 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 the nitrite toxicity. After the initial application, the dosage applies ONLY to the amount of water being taken out and replaced, NOT to the amount of water in the entire pond, and NOT to water being added to replace that lost by evaporation. Except for very short-term medicinal baths at concentrations often around 25 ppt (1 pound per 5 gallons), and administered under tightly controlled conditions, it is not recommended that Koi be subjected to a salinity exceeding 5 ppt (4 pounds per hundred gallons), especially for extended periods.
Salinity levels are normally maintained by the addition of salt to increase it and by water change outs to decrease it. Introduce the salt, if possible, at the discharge side of the bio-converter (not at the bio-converter inlet nor directly into the pond). 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. When making the initial or any large application, it is probably better to divide it into two to four daily partial additions rather than putting it in all at once. Inexpensive and quite pure solar-dried or kiln-dried salt used in home water softeners is available at most supermarkets and home improvement centers. Do not use pelletized water softener salt that has binding agents or any type of iodized salt.
The floating hydrometers that are used to measure the salinity of salt water aquariums will not supply the accuracy necessary for use in a Koi pond. Electronic conductivity meters will give an indication of the amount of salt but can give false readings due to other substances in the water. A chemical test kit is available from LaMotte that is designed to measure 0-20 ppt. By increasing the sample size by four and dividing the reading by four, the kit can be used to measure our desired range of 0-5 ppt. Aquarium Pharmaceuticals also has an inexpensive salinity test kit available that is quite accurate over the designed range of 0-2.4 ppt. The water sample can be mixed with an equal amount of distilled water and then multiply the reading by two to extend the range to 0-4.8ppt.
A salinity test kit is not considered to be a requirement for the average pond but one should be used if attempting to maintain salinity levels above 4 ppt.