I also keep gecko's and snakes and use the under tank heaters. I have
never had a bit of trouble with them either, but I have to agree that
they do not put out enough heat for the typical tropical fish. Try some
of the fish that like cooler water, unless you want to pick up a small
heater for your tank.

Just make sure you wash the things that you had in with your gecko very
well before you add them to the fish tank.

Vicki