Stronger salt baths of 2.5% for 10 minutes, can quickly rid a fish of many parasites and bacteria or fungus. However, when the fish goes back into the pond, it may still be in a weakened condition and might fall prey to a new parasite. This is why it is necessary to medicate the whole pond properly. Parasiticides or antibiotics may need to be added concurrently with salt treatments. Concentrations of salt stronger than 0.3% combined with formalin may be too harsh.
Salt has been used to control string algae at a concentration of .25% and higher. Killing large quantities of algae with salt may pollute the pond. Remove most of the algae by hand fisrt to reduce pollution.
Salt concentrations of 0.3% are effective at detoxifying nitrite. The salt interferes with the nitrite ion exchange at the fish's gills. Use until nitrites have cycled properly.
A 0.3% concentration is a good all around tonic for strengthening koi and improving disease treatments. This concentration will stunt your water lilies and stringy algae. You may choose to treat your fish in a separate container.
A 1.0% salt concentration is the same salinity as fish blood. This is called an isotonic solution. Salt does not transfer into or out of the fish's body. Under normal pond salinities (0.1%) ulcer disease can cause a loss of internal salts through the open sore. Addition of 1.0% salt to the koi in a separate hospital tank will limit this loss of internal salts. Limit this concentration to 2 to 4 weeks.
Always adjust salt levels gradually over 3 days to allow the koi time to adjust. Do not use salt continuously as parasites can become immune. The beneficial effects will be diminished. Use salt in the spring as a preventative for disease or when the koi are definitely sick. Monthly water changes will dilute salt to normal over time.
Note that the salt that should be used is non ionized, often refered to as Ice Cream Salt or Rock Salt.