So I just set my my tank for the first time in years. It's just a little ten gallon. I used a filter given to me from a friend of mine who changed tanks, so the filter already had some good biological stuff. I got four neon tetras and resolved to change the water very often since they're somewhat delicate. Well, after just a few days I saw it. Yup, the dreaded white spots of ich. My pet store is very good and agreed to take them back and treat them there, but now I have this tank with ich and I don't want to use chemicals. I've got the heat up to 32°C (90°F). There are no fish in it. How long should I leave it that way before it's safe to try again?