Congrats on the new frog. I have just read a care sheet on tomato frogs and there requirements almost seem the same as pacman frog, which I have. So ya you could use a undertank heater but make sure u place it on the side or back of the enclosure. Not on the bottom because he wont be able to figure out why he cant cool off when he is burrowing. And I find that uth's don't evaporate the moisture as much as lights so u don't have to mist as much. And you can use both light and uth or an uth and a ceramic heat emitter. Hope I could help.