It slows growth rates to a crawl. I have a 3 gallon next to a window with no heater or light and temps run between 66-68 right now. Breeding has stopped and babies don't grow. They are healthy, just slowed growth. Activity is normal but the tank runs cool most of the time so I wouldn't see a dramatic difference. I've had a heater on it in the past and didn't see a giant difference. I have another tank away from the window with no heater and sits between 68-70 and growth is slow on babies as well. Tanks house RCS/CRS and RCS/OEBTS respectively. YOu can do it just fine, depends on what you want to do. If you want them breeding and growing fast a heater in the winter months might be advisable.