The otos love zucchini and/or cucumbers. If you slice about 1/4", microwave it in hot water for a minute, soak it in cold water, then sink it in the tank with a small bag clip, your otos will be happy to chow down on it for days. That's what mine love. Plus, the zucchini won't pollute your water quality nearly as much as algae wafers can.
I've got two otos in a 10 gallon (started off with three) and they've done quite well with me. I lost one of them early on but have had the others for almost a year. My fish have to be hardy too because I go to college and bring them back and forth from my apartment back home a few times a year.