Their proper diet is vegetable matter, and they supplement that diet with algea. If they are left to eat just algea and whatever the other fish leave laying around, they can get intestinal problems or starve. So keep giving the little stinker his vegies, and he'll love you for it!
Really? I didn't know that vegetable matter was grown in the rivers and streams of South America.
All joking aside, they need protein in there diet too. Bet you need see a BN pleco turn up his nose at blood worms.