My catfish stay outside year round here in North Texas. With that said, when checking temps pay attention to water temps, not air temps. Water here is typically about 7-10 degrees cooler than air temps when looking at lows. Deep, slow moving water holds heat longer than fast moving shallow water. Heater may not be a bad idea, even if a small one...even if not needed. Higher temp means higher metabolism, which means faster growth and feed coversion. Also increases possibility of breeding. But it does decrease DO. I guess whether or not you need a heater depends a lot more on how big your water volume is, in the ground or above ground, indoor/outdoor, etc. And...how big your cats are, as little ones are more sensitive than older. However, I have fished cats out from under ice on the St Lawrence river in NY and they were plenty live till I ate em. So, I think lower temps would just slow them down, and slow down consumption of food. Ice doesn't seem to be a problem, as long as not completly frozen over. DO is a big issue. If not a lot of water, try bullhead. Best of luck.