I bought 5 Otos about 6 months ago for my 50L. I'm now down to 4 but it's more than likely because I moved house and had to set the tank up again.
At the start I didn't feed them that much as there was so much algae, but I soon found out that there's only certain types of algae that they eat. They ate all the algae they could, then then started to eat my shrimps!
When that happened, I started giving them Zucchini/Courgette. I slice it all up and put it in the freezer, then take it out and microwave it for 15 seconds and then put it under a cold tap until its cooled down. I highly recommend you do this else you'll be wasting a lot of money buying new food. You can keep it frozen for about 8 months, if you don't, most courgettes only last a week in the fridge. I feed them about 1 slice per week. Find a way to put it as close to your filter intake as you can because the stuff disintegrates really quickly, you don't want it floating onto plants/rocks.
I too have some HC. Unlike Corys, they don't dig about too much and wont really disturb it. They're quite gentle when they do. They've managed to uproot a bit in the corners of the tank and at the front of the glass where they swim past, but if you trim the HC back a bit it will give them enough room to swim by without touching it.
Hope that helps.