Gravel is no good, but rocks that are too big to be swallowed are fine. The one mat heater I have is mounted on the side of the tank and it works fine. As far as the worm is concerned, I would clean out that bowl and take him to the vet for a deworming. I am assuming that your frog is wild caught and most likely has gastrointestinal parasites. Have your vet treat the frog with metronidazole for protazoans and fenbendazole for worms. The two drugs should not me administered at the same time. Treats? Moths, treefrogs love moths. Get some waxworms and let them mature into moths. Keep them in a clear jar so you can monitor their progress and make it easier to harvest them for your frog.