I am currently using this heater in my 300gal Tilapia tank and the water has been staying around 80-85.

http://www.amazon.com/Farm-Innovators-Su...farm+innovators

If you can figure out how to rig up some sort of thermostat I think you would be fine. That is the only issue I have.

I used it in my 100gal grow out tank as well when we had those early cold days but the water would get close to 100 in it.