We have one of the black tube type submersible, Aquaguard or something like that. I keep it in the back section where the filter is. The water level in the pump section flucuates so much that i was scared it would burn up the heater. Just make sure to get the one that is short enough to fit in that section with most of it underwater. I want to say it is a 25 watt but I could be wrong. Whatever it is it holds the water at 78 - 79 degrees day and night.