I maintain 74 to 78, in reality I don't know my lower range, I've kept many tanks without heaters in San Diego and the bay area and have never lost a coral to low temp that I know of, so I would guess a low of around 70/72 in the winter. I've also been up to 93 before, surprisingly didn't lose anything either. Went on vacation and accidentally turned my light timer ahead 4 hours instead of back, so my halide and mhos turned on at noon in 85 degree weather instead of 8... Everything established was ok, cuc wasn't happy and all of the $200 in corals I brought back died, even after acclimating and waiting for my temp to drop to 82. It really seems to depend on what the corals are used to so I would recommend dropping the tank 2-4 degrees a month till your where you want and have consistency in daily fluctuations to not shock them.