Last year I purchased several nice big otos for my 65g. I figured I would give them nori when they ran out of algae. Here's the thing; there's no visible algae in the tank, probably due to tannins limiting the amount of light, but the otos still look fat and happy. I've given them nori a few times and they seem interested, but they don't swarm it or act starved. Is it possible they're eating just the bio-film and doing fine with that? They still have their plump tummies... I'd show a picture, but they move around whenever they see me.