I just had a pleco die and tested the water--it was over the top on nitrates. I took some water down to the fish store and had them check it too( which most of them don't charge for) They found it over the top for nitrates too. They had me take out about half the water and come back tomorrow. I'd say check the chemistry first before you do anything. It could be the beginning of a problem that is pretty easily resolved in the end.