First estimate the actual water volume in your water section by using this formula or any other on the net: Aquarium Volume Calculator (W.C.S. Ltd.) . Once you get approximate gallons can get a heater rated for that volume. Usually they are rated to raise temp. 10 degrees F above ambient. I prefer Eheim's Jagger heaters because of their quality, shatterproof glass, accuracy, and efficiency; but as long as fully submersible you should be OK. Also, locate so frog does not come into contact with it. Good luck!