I have had 2 common Plecostumos'. 1 was 7 inches long and the other was still young. The larger Pleco never bothered my plants. If they are getting enough other foods, they shouldn't bother your plants. I feed mine Brine Shrimp, Vegetables, Blood Worms, Algae Wafers and never had an issue. Having a good variety of foods keeps them happy also.