Try feeding a variety of food is the plec is causing problems.
I vary food from frozen bloodworms, brine shrimp, daphnia, vegetable, flake food, sinking pellets and hikari algae wafers.
The algae wafers are great for your plec and will keep him busy whilst you feed your other fish something different. I tend to drop in an algae wafer for my BN, some sinking pellets for the corys and flake food for all the other fish. This is only on the days I feed flake food, the other days when I feed bloodworm etc he seems fine eating that.
Andy