Mine did the same thing about 2 months ago. Thought i was going to lose him. My wet dry crashed... But I bumped the temp up 4 degrees and did 25% WC twice a day for 7 days and he was all better. Looks like Ammonia burn to me. Keep changing the water he should pull through. Good luck hope it works out.
Well he looks the about the same swimming around and breathing fine water test good I going to keep doing changes about 2 to 3 a day but what about types of meds? anti bac? what types if any? Will his skin grow back alright? what level of scareing he doesn't seem to be in any pain anymore