I also got over a case of ich recently. I used salt and raised my temp control from 80 to 84 in one step. It took a little over 24 hours to reach the final temp, and my fish were fine. If you do the same, I imagine it would also take enough time where your fish wouldn't suffer a temp shock. I believe it has to do with something called the "specific heat" of water, which is pretty high compared to something like metals. This high SH means it takes a while to heat water to a certain temp (assuming you don't use an overly intense heat source) and it also takes a while to lower the water temp to room temp, if your heater failed for example. Anyway, it also depends on how much you want to raise your temp and probably depends on the wattage of your heater in relation to the volume of your tank. Oh yeah, and what kind of fish you have, and the temp range that they can tolerate. If they are used to 75, I'm not sure if they would survive an increase to 84.