The use of therapeutic salt treatment has been widespread through the history of the koi hobby. In the early days it was used as a cure-all, panacea type of treatment for all sorts of ailments, from parasite infestations to bacterial infections, when no other drugs were available. Many koi keepers still maintain a constant dosage of salt throughout the year, or add salt in spring even though their koi are healthy. This practice troubles me and I will attempt to explain in this article.
It happens that koi, and carp in general, have a relatively high tolerance for salty water. This tolerance allows for a koi to be placed in water that is too salty for parasites to live in, yet not salty enough to kill the koi. In the relatively early days of the hobby, Japanese farmers exploited this characteristic and used salt treatments to cure koi of parasites. These treatments worked well in those days for parasites such as Ich and Chilodonella. However, parasite adaptation being what it is, those which were susceptible to the treatment gradually built up a tolerance for salt, and in modern times, in-pond salt treatments are nearly ineffective for any sort of koi parasite. The levels of salt now required to eradicate common koi parasites are so large that they approach the threshold that could damage the koi if left in the treatment for an extended period of time. Short-term salt baths still retain some effectiveness as they are used in high concentration and for only a short time, lessening the potential damage to the koi.
Salt, though tolerated by koi, is an irritant. It causes the koi to increase its mucous production to insulate its body from the irritant. My concern is that a koi that is constantly subjected to this low level irritation by long-term salt "treatment" is less healthy. My personal opinion is that a koi that is constantly producing mucous to react to salt is less capable offending off other stressors, when they arise.
Salt has also been used by some as a treatment for bacterial infection. This strategy is dubious at best. First of all, the most common bacteria affecting koi, Aeromonas, is not affected by salt levels in which koi can survive. If a koi has an infection, do not rely on salt alone to treat the koi.
That being said, salt does have valuable uses in our hobby. When a koi has been in a show, or recently shipped, the use of salt, as an in-pond treatment at around 0.3% concentration is effective to increase mucous excretion and help to heal transport/handling abrasions. When used for a short period of time, salt will assist stressed koi as they recover from disease. This would be one of two situations in which I would recommend an in-pond use of salt. Important also is to flush out the salt through regular water changes when the treatment has run its course.
The second condition in which I would advocate in-pond salt use is to combat "New Pond Syndrome." When a pond is first filled with water and koi are added, it has no bacteria in the filter to break down ammonia produced by the koi. This typically results in a rise in ammonia levels in the pond until the filter "kicks in." When the bacteria in the filter begin to colonize and break down ammonia, it is converted to nitrite. Nitrite is equally dangerous to koi and is also eventually removed by filter bacteria. Nitrite-reducing bacteria are notoriously slow to populate koi ponds, and the interim time of high nitrite levels can prove hazardous to the koi. Salt is effective in this situation to counteract the effects of nitrite on koi. A low dosage of salt (about 0.1%) is all that is needed to ease your koi through this potentially dangerous time when starting a new pond.
Another key drawback to keeping salt in your pond is that you are not able to use other pond treatments at the same time. There have been cases where grave damage to koi collections occurred when they were subjected to treatments such as Formalin while salt was still in the pond water. Combinations of any pond treatment should be avoided if at all possible. So, considering the time it takes to effectively reduce the salt concentration via water changes, you increase the time you must wait to switch to another treatment such as Formalin/malachite green or potassium permanganate.
My general position on the use of koi as a health treatment is "Why use it if you don't need it?" There are only a couple of valid reasons for the use of salt in your pond, and if you don't fall into one of those categories (new pond or recovery from damage/transport), I'd discourage its use. There are more effective treatments out there if your koi have a problem. The next time you are faced with a koi health problem, explore the treatments available before habitually reaching for the salt bag.
Proper placement of pond de-icers is crucial to optimum performance. Make sure you follow the manufacturer's instructions. The de-icer should generally be placed at the edge of the pond in a shallow area.
