well, theres always the algae discs, cucmber/zucchini, that sort of thing...........
i dont know anythingn about rusty plecs in particular, if theres anything else they might like, but most plecs enjoy the above...............
Watch the tank after lights out and see him go!! My plecs all loaf around doing very little during the day, but once the light goes out, they are furiously looking for the spoils of the days feeding!!