Well, I found out why the wall sucker was being tortured. Its also the answer to every other question in the universe. Babies. Evidently some of the striped fish had babies and they were paranoid that the big different fish would eat their babies when hes really a harmless soul who only eats nutrients on the wall of the fish tank. But now that the babies are big they don't mind him being anywhere besides that wall, they just ignore him.
One question (that I didn't state earlier) remains; why do the striped fish chase eachother? I think they're training for the potential wall sucker threat, although he poses no threat at all. If only we all could be understood.