I have 2 albino BNs (which are omnivorous) and a Sultan Pleco, which is (mostly) carnivorous but does take the occasional algae pellet. A Pleco, or any fish for that matter, will never turn down a free and easy meal. Plecos, in general, are not predatory, and even some strictly carnivorous plecos, like a zebra, or a sultan, will not hunt and kill. I parrot what everyone else has said about needing the chemical levels. Bio load, ammonia level, or if the fish are new could be an issue... Also, ich -- if gone unnoticed and untreated can demolish all but those heavily armored plecos and a dead or dying fish is, as I stated above, is a free and easy meal.