I was talking to Butterfly and she said a lack of protein in the pleco's diet may cause that pleco chasing fish thing. Plecos suck on the fish and leave marks because the slime coat is pure protein. So you could try feeding a protein rich food one day a week, the rest of the week a veggie diet would be best.
Butterfly is such a smart woman, that's why we all love her.
You could try Bristlenose Plecostomus, but make sure it is near full size or at the very least larger than the oscar when it is put in, to avoid the oscar devouring the pleco.
