How often do you do water changes? Getting rid of the platies isn't going to make much difference on the bioload as the common pleco is the real poop machine. He's going to need a 125 gallon tank and you have to move him as soon as possible; has the pleco been in there for 3 years? If so it is most definitely stunted but it may still be reversible. I would suggest you start by doing a water change to get the nitrite down. Are you using test strips or liquid test kit? IME liquid is more accurate.
