Even though other fish show no signs of it, it takes a while for the parasite to form the white cyst, and they may already be infected with it. To treat, without Ich medications, I have successfully used aquarium salt as a cure. Keep the temperature raised, and use aquarium salt, which can be purchased cheaply at an aquarium store. Use the recommended dose on the package. Mix the salt first with removed tank water in a clean container, and then add small amounts of that over a few hours, which will help keep the fish from stressing out. Do NOT add more salt -except- after your scheduled water changes. Only add salt for the amount of water you are replacing, not an entire dose for a tank. Salt will not evaporate out of the water, so do not add any salt when replacing water to tank from evaporation. DO use an air stone with an air pump, salt will lower oxygen levels in the water. Keep up regular tank mainentenance and water changes.
The salt will not kill the ich while it is on the fish, it will kill it once the cyst bursts and the parasite multiplies re-entering the water. Many parasite will find its way into the gravel, so it is important to regularly gravel vacuum extremely well to remove it. Keep up the salt treatment until at least ten days after the last spot has gone from off the fish.
To remove salt from the water, simply do regular water changes, and add no more salt, this will slowly over time remove the salt, and not stress the fish from to sudden of a salinity change.
I have successfully used this method when my neon tank broke out with Ich after a new neon was added to the tank without quarantine (big mistake). I did not lose a single fish.
2 10 gallon tanks, 1 20 gallon tank, 1 Fluval Edge, 1 29 gallon tank, and one backyard pond.