You may not feed your pleco and think that he is living off of algae, but most of the time that isn't true. Lots of times they will eat flakes, granules, ect. that fall to the bottom. Most plecs can survive off of algae when they are young, but as they get larger you should throw a little extra food in for them to eat, they can eat algae very quickly and end up having no food source. For instance when I put my plec from the pond to an algea covered 29 gallon he will completely clean the whole tank spottless of algea within a day or two.