I haven't had complete success feeding them, though. Since I moved Mavis out again, I'm also am opening curtains in the flanking windows so the tank gets hit with a bit of indirect daylight and there's some algae growth on the end sides. I put a zucchini slice in once a week (day before water change in case it fouls the water too much)- my four-year-old says it looks like I'm "giving the fishies a popsicle". They never seem to eat much of it though- I keep an eye on them all day and only once or twice see an oto on the zucchini, not for long. It does always cause the glass to become cloudy with the start of algae bloom, which the otos happily scrape off in the next day or two. So at least that is keeping them fat. (Typically, according to other aquarists, a ten gallon tank can't harbor enough algae to keep otos from slowly starving. But I don't want to encourage so much of it that it gets unsightly, either).
I put aquarium river rocks in tank water with drops of flourish comp, in windowsill, in case I can grow a film of algae on that for them too.