Regular aquarium heaters. I don't see a need for a heater just for Critter Keepers ? As long as it fits, it should be just fine. The only thing that would bother me is the hood, but that's just my opinion. I have
to have a tank with a hood and
light attached, haha ...
The only "different" thing about critter keepers is that they are plastic, as opposed to glass or acrylic. If it's a 5 gallon, it will be 5 gallons, so everything you can put in *this* five gallon you should be able to put in *that* five gallon, unless it's a hex and a rectangular in comparison.
Heaters are necessary with bettas, so if you can't stick a heater in there, then don't get it unless it can be properly warmed up. But by the looks of it, I'm pretty sure this one can heat easy.