Ich stays on the fish till its ready to multiply then they fall off and fall to the substrate, each speck that you saw on your fish multiply in a cell until it bursts and then these free swimming parasites emerge and start looking for a fish. They have a short life span in this stage. If you can keep your fish healthy enough to keep the parasites off of them, the parasites wonââ"šÂ¬Ã¢"žÂ¢t be a problem. I think increasing the temperature does reduce the life span of the free swimming parasites, by increasing their metabolism also increasing the photo period does something to help. Maybe because the fish are moving around instead of sleeping in the rocks makes it harder for the parasites to attach. I donââ"šÂ¬Ã¢"žÂ¢t know.
In my tank I always put fish with ich in the tank and never had a problem, the ich goes away by it self. Donââ"šÂ¬Ã¢"žÂ¢t ask me how. I have cleaner gobies and my fish are kept fat. I go through a lot of food. Healthy fish = no ich.