^^^ I also don't feed my common pleco anything special. I notice he comes out after feeding and picks up flakes off the substrate. He does not do much algae cleaning though, lazy fish.
My bristlenoses (tiny 1"-1.25")in my other aquarium are constantly scavanging, night and day, eating algae, leftoverflakes, whatever they can find. I feed them an algae wafer treat about 2x a week.
So, I've had good experiences keeping them alive either way. The only time I've ever lost a pleco was when one jumped out of my first 10g tank. How do the other fish act towards it? Can they be being picked on to death?