Aiseh...common pleco also got problem ar ?
my common stay inside my side filter ler...u can ask joe(cichlid connection) to confirm...he saw it
Has been in there for a long time...also no problem in fact he is getting well fed in there.
Generally boils down to bad water condition...usually commons can stand very bad water conditions...but ur tank must be in a more terrible state...LOL
Just kidding...
Most probably it is already infected when u got him...if u still wanna rescue him, but i doubt that it could work, depends on luck...change water regularly for everyday or every two day for the next one week.
If do not help, then get a new one