its common with pacu. chances are, its not mouth rot at all, its from him smacking into the glass. i have 2 big pacu, 1 of which his mouth is ALWAYS messed up to the point you can actually see his teeth, because he's the skittish one thats always smacking face first into pots and the glass. he's been that way for close to a year now, yet still eats like normal.
as long as your water is clean, and you keep up on changing it, you should be fine. i worried like hell at first, until i figured out what it was from.
