mebbe use less? i've found also that feeding the main fish in the tank first and shutting off the lights and then dropping the pellets/wafers asap near the plecs fav spots tends to reduce other fish from going for them right away.
algae pellets when uneaten basically dissolves and turns water quality bad. its easier to use veg, and take it out after a few hours, before it destroys water quality. i think its better for the plecs too imo.
i slice the cucumber, and just put in a few slices, the plecs eat from the middle outwards. im sometimes left with a few little discs with holes in the middle the next morning. never had a problem water quality wise