Ok so i have a 20 gallon high with 3 fire-belly toads in it. I would like to raise the water up half full(or however full i need it) and put a few fish in with the toads. Is that ok to do? I already have a filter system for the water, so all i would need is a water heater for the fish. I'm not sure what species of fish to put in yet, but there will only be 2-3 of them. Will this work?