The tank parameters arePersonally I wouldn't worry about the pleco hiding. Plecos tend to be more nocturnal anyways. As for the other fish it could easily be fin rot but it could also be aggression amongst tank mates. Tank parameters are going to be a must to rule out fin rot or any other diseases. It would also be helpful to get a pic of the whole tank along with meserments of how large it is. Also if you don't mind what is all stocked in the tank?
Ph 8.0
Ammonia sorry I couldn’t test it
Nitrite 0
Nitrate 30
This is with me doing a 60 percent water change five days ago.
They’re haps and peacocks I believe.
