Hello ali1, give Marine-Max by Tropical Science a look. I use it as intended (prevention) but have dosed it when the fish already had ich and the fish were able to ward it off. It is an immune system booster. The fact that if the fish are able through a healthy immune system to ward off ich, the ich will have no effect on them. The proof is this; ever notice or heard that only some fish in the tank get it and others don't? The fish that don't have a stronger immune system. Check it out, I think you will be happy with the results.