A sunken belly usually means that the pleco is too far gone If they have a sunken stomach often they will go off their food and die even if food is plentiful. Is the pleco eating at all?
What medications have you got in there? I know its an odd question but you don't have salt in there do you? As plecos are very suceptible to salt.
is salt really harmful for plecos? i have had several ( just common ones ) and, not often,but if im medicating or something happens that stresses my fish, whatever, i put some salt in there, i havent ever had a problem with the pleco, but im glad i know this now. what exactly does it do to the pleco that is harmful?
Well I'm not the catfish expert so hopefully one of them will come to advise more but I think it reacts with their coat which is smooth skinned - no scales- resulting in burning, similar to putting salt on a slug. As I say best if you post in catfish cradle to get a bit more specialist help.