Mine like vegetables a lot, zucchini and cucumber seem to be their favorite. Algae wafers are good too but my ancistrus clean those up before the ottos get a chance to see them. I usually attach a half pealed zucchini with a veggie clip in the evening and it's all gone within 24 hours, but I have a lot of anchistrus now, I used to use slices before they multiplied so much.
Some of mine will eat zucchini others won't; some like algae wafer. One thing that you should be aware of is that while they eat algae they also need protein so add some shrimp pellets or flakes or something other than just vegi. Normally they eat a lot of biofilm in addition to algae but in a new tank neither will exist. I had one very hungry otto who used to float upside down at the top of the tank and suck in flakes when i fed the guppies. Was kind of cute but when he got fat he stopped doing it.