I should have said; it's not a necessary part of their diet, although some species of plec are more carnivorous than others. You shouldn't be feeding it more than once a week.
Most fish love to eat fish; the rule is you feed only sea fish to freshwater fish, and freshwater fish to marines (because you're feeding it raw, there is tiny chance of introducing parasites or diseases, but marine parasites can't live in freshwater, and vice versa).