When i got my first frog, he burnt himself when i was cleaning his night light cover (really stupid on my part, I know, i felt awful for ages) and he got a nasty sore on his back. It didnt look like the sore on yours though, so it is obvious yours isnt a burn. Sorry i am rambling, the point was, the vet told me to keep the tank really clean to stop the build up of infection on the sore. Might help you, and good news, he lived!