Honestly I've never tried it, but I think it could work as long as it is a very reliable heater such as the Visi-Therm Stealth. I have however used a 50 watt Visi-Therm Deluxe in a 5.5 gallon with no problems whatsoever, and I'm currently using it in my 5 gallon.
I'd say try it just keep a close eye on it for a while. You can always bring it back if it causes you problems.
Edit: HOLD ON A SECOND! I see a pic in your avatar, did you get a camera? If you did, we NEED pictures of your tanks and fish!