Hello,
I will see if I can remember what I had mentioned.
I think I was asking if you had anything in the tank such as a heatrock that could have burned him, or is your undertank heater getting too hot perhaps?
You could drain the area, but you should be careful with the skin as it is pretty fragile & you should have a disinfectant such as betadine handy to clean it in case any blood leaks out.
Has the area improved any at all right now?
It is affecting his behavior at all & is he eating normally?
Does it look like a bite caused from a cricket perhaps, or not?
Tracie