Ich is a ciliated protozoan that can kill smaller koi by damaging their gills. It can be a real irritant to larger koi and can cause secondary bacterial infections. In tropical fish and gold fish you will see small white spots on the scales of the fish caused by the Ich parasite under the scale. These parasites go through a life cycle that averages about 2 weeks in duration. Other diseases can cause the appearance of white spots too, so you can’t always be certain that you are looking at ich. Those white spots so well associated with Ich seldom if ever show up on koi infected by Ich. Under the microscope Ich looks like a large, slowly rotating, ball with a “U” shaped nucleus inside. Ich encysts under the epidermis layer of the skin of the fish. It will live there, depending on the temperature of the water, for up to 5 weeks in cold water. Medicating the pond to kill the Ich will have no effect on the parasite until it ventures out from under the skin to reproduce. At that time salt at .4% will easily kill it. For that reason you need to salt the pond for an extended period of time to effectively kill ich.
Salt when used to treat for ich should be added at a rate of 1 pound per 100 gallons of water each day until the pond reaches .4% salt. If there is no salt in the pond to start with, this will take 4 days to reach .4%. A 1000 gallon pond will require 40 pounds of salt to reach a salt level of .4%. This may sound like a lot of salt but if you add it over a 4 day period it will not bother the fish or the filter. Leave the salt level at .4% for at least 3 weeks. Ich is easily killed with salt when it come out from under the fish’s scale to reproduce, however it’s life cycle is such that it could take up to 3 weeks to come out and be exposed to the salt water. You can purchase 50 pound bags of solar salt at Home Depot for around $5.00 each. Use the cheapest salt you can find because it will have no additives in it which could be harmful to the koi.