Otos are pretty small, and very clean. They grow to about 2.5".
They are notoriously picky about water conditions, though.
They also need a constant supply of food in order to stay alive.
They are shy and like to hide under wood, in caves, or the underside of broad leafed plants.
My otos usually eat algae in the tank, plus I drop in algae tabs for them and they will occasionally munch on the sinking wafers for my cories, so giving them a little protein is a good idea now and then.
Amano shrimp also will eat algae but your discus might also eat them.
Other fish, such as plecs or chinese algae eaters grow to be large, messy and don't eat algae as a primary food source anyway. They might eat algae, but only if they are young or have no other food available.