It is essential to know what kind of 'pleco' you have in order to provide adequate nutrition. For carnivorous fish, providing meaty foods is very important to their well-being. Carnivorous fish fed only plant matter will not get adequate protien and will suffer in the long run. Carnivorous fish often can not adequately digest plant matter--they lack the needed bacteria in their gut.
Blood worms, glass worms, beef heart, brine shrimp, mysis shrimp, meaty sinking tablets (Hikari makes one that is specifically for cats that has lots of protein), shrimp pellets, and community formula frozen foods will all help provide the fish with meatiness they need to thrive. Blood worms are available frozen--these are easy to feed, but shouldn't be the only meaty food you supply.