Yes, they will. But how much you give them depends on the species. Some plec species are more or less carnivorous (e.g. Acanthicus adonis) and will do very well on a mixed diet of prawns, mussels, and bloodworms. Others are almost entirely herbivorous (e.g. Panaque nigrolineatus) and should receive mostly green foods (and wood) with meaty foods as only an occasional treat. Common plecs (e.g. Pterygoplichthys spp.) are somewhere in between the two, and need a mix of meaty and green foods.