Any tankmates that could possibly be picking on it?
If not, I have to agree with the others that it could very well be bacterial, as they are very susceptible to such. I went through a similar situation and was unable to figure out what happened. I was equally freaked out and even wondered if the one (i also had two) had attacked the other one and caused the damage as it did not occur until the day after they "found" each other after days in the tank. Rationally thinking about this, I discredited it as then there would be tons of this occurring at LFS
holding tanks! Possibly, whatever happened with the water change cause some stress in the tank...and this weakens them just enough to being exploited by the bacteria.
I would caution against moving them unless the tank water is exactly the same. I attempted to move mine and even in a drip acclimation it made things three time worse, three times as fast. I don't think there is much that can be done but keeping very stable water until it heals (i encountered someone else this happened to and theirs almost fully recovered before "crashing" totally.
Here is a picture, I bet we are speaking of the exact same thing: