Actually, they (Chinese firebelly's) were both rescued from a school which had a mixed species set-up when the now deceased became ill. They were both put in separate isolation tanks at my home. This one-male- was only recently reintroduced to a larger tank that was properly set-up for newts just before I noticed the sore.
There were snails in the previous tank... and although I thought I'd removed them all from the plants for the new tank, I just found one 1mm sized one (near where he slept last night)... Maybe the sore was caused by the snail? or maybe it's rot?
I tried dry quarentine for the other and it declined very rapidly (I may have been just too late though). I've put melafix in his water... and was planning to adjust treatment tomorrow depending on the progess of the sore. The sore is currently a pinhead sized hole and it appears to show bone. It's located about where the ears are on our lizard.