Otos pretty much eat and poop constantly. They'll only eat what they need and because they digest so fast and eat only algae and veggies they won't get constipation problems like carnivorous fish.
Wash some spinach or Romaine lettuce really well and put it in the freezer for an hour. Then take it out and put in the tank with a veggie clip made for fish tanks (you can get these at Petsmart or your
LFS) I think you can leave it in for at least 24 hours. If they need more food they will eat that. They also like cucumber or zuchinni.