Actually the temp should be turned up to above 86 degrees over a 2 or 3 day period. Just turn it up a couple notches in the morning, a couple more in the evening, and do again the next day, and a 3rd day if you want to spread it out a little longer. Overnight is actually too fast, IMO. But I've seen my tank go up past that in a day without stressing the fish too much when I accidently forgot to turn the A/C on before going to work. From what I've noticed, you want to turn the temp down slower than you turn it up, as going to colder temps seem to stress the fish a little faster than warming them up. But you should do it over a 2 or 3 day period. I use 89 degrees myself for treating ick, and run an air bubbler. And my reasoning for 89 degrees is so the minimum temp doesn't go below 86 degrees when the heater turns on and off.