The only problem with 90 degree's is potential sterility of fish in the aquarium. And yes, I understand that some fish can, and will, withstand high temperatures because they have evolved techniques to deal with that issue in the wild. Prevention is to have healthy, stress free fish, non-major temperature swings which causes stress. The higher temperatures, to @84 will kill the ich-cycle--if kept at that temp for 3-4 days and a slow resumption of the 'normal' temperature. Anything that opaque the water--like methlyene blue is helpful in reducing the ich reproducing process; but silicone is blued, too. Some of the ich-medicines are useless; others work, but also have the potential of killing some fish, killing the patients, and one's patience. I typical rung tanks without heaters, no filtration--only room temp (55--75), water changes of 50%. Have not had a case of ich in 17 years in any of my up to 100 tanks (20 now, in an apartment). >