A well balanced diet, for a plecostomus, consists of algae wafers supplemented with zucchini or cucumbers once or twice a week. The raw vegetables should be anchored near the bottom of the aquarium, an algae clip is a good choice to achieve this!
Up to 12+ inches depending on the species.
Plecostomus are primarily nocturnal and best to feed prior to turning off the lights for the evening.