They eat things other than algae. Diatoms are one of those things.
Just keep an eye on those round bellies and feel free to supplement if needed. Otherwise, sounds like you have happy otos, congrats!I do have a bunch of oak leaves and sticks in there... I bet there's all kinds of stuff growing on there.
Cool! I have otos who don't need extra feeding. I'll still put nori in for the tetras, but it's good to know that my ottos are going to stay fat and happy.