I have telescope eyed goldfish and fantails. They tolerate heat so well (contrary to popular belief) that I switched from salt and some heat to heat alone. 86 degrees stops it from reproducing or reinfecting. 88 degrees kills it outright, but not the stuff on the fish which takes a few days to mature out then dies once it drops off the fish. I turn the tank up to 90 degrees which, in my setup, requires a 300 watt Eheim heater in the 15 gallon sump of my 80 gallon tank. The ick is dead and gone in about seven or eight days. I leave the heat up for at least ten days to be sure. I have never seen this kind of fish show any stress from 90 degree heat. Apparently sustained temps in the mid ninetys can be bad, but not 90. 110 is lethal. These fish love the heat at 90. My heater takes an hour to raise the temp 1 degree. I turn it on full blast and it allows plenty of time for acclimation. I have hear rumors of a strain of ich in florida which requires 92 degrees to kill it.
I sometimes add new fish without quarantining. These fish often bring ick which often appears on the old fish but not visibly on the new arrivals. The heat wipes it all out for good. If your Ick is reoccuring your fish have a weakened immune system. I am told that Ick in fish is a bit like Strep in humans - always there in small numbers but kept in check and harmless as long as you are healthy. If the immune system is compromised or something causes the ick concentration to rise it becomes dangerous. Your fish may have an internal parasitic, viral or bacterial infection or some other thing which is predisposing them to the ick. The solution there is hard to pinpoint. If it kept coming back I might guess bacterial problem first and try a course of food based antibiotics followed by probiotic gut recolonization, but as a last option.