You still need to use the heaters. If the room temp decreases on a cold day, so will the tank temp. I made this mistake on my 75g. I use a big Mag drive pump, and it always keeps the water in the lower 80s. I left on vacation for a week and someone decided it would be okay to turn the thermostat down. I came home to a 66 degree tank full of wild caught Geos. I feel that I was very lucky when none of them got sick.