On second thought I would first suspect some stressor you are inadvertantly causing. Any stress wears down the immune system of any creature; slime coat is the first thing to suffer in fish and ich is often the first opportunist. I used to be pretty careless about the temperature of my water changes. I know people who have incorrectly assumed they had no chloramine in their water and used half doses of reducer. Toxic tank decor. Nocturnal O2 deficits or pH swings. Crappy, tainted or expired and rotting food. Voltage leaks from bad equipment. rapidly fluctuating temps from drafts or heat registers. Some oddball parameter out of whack. Any number of invisible things can slip by our notice and cause stress. Even a bacterial infection should clear up by itself if there are no stressors. If the Ich keeps coming back I would think very hard on everything you put in the tank before guessing at and treating for stuff blindly. I wish I could say bombard it with KILLALL!! brand multi toxin and let god sort out the dead bugs but that stuff seems to beat the heck out of the already weak fish IMO/E.