Don't use cucumbers, eggplants, or bitter melon, winter melon, watermelon, etc...
I buy Zucs when they're like $.50/lb or less, yellows or green would work. I would only leave the thin slices in the tank for 12 hours MAX before I remove them. You may want to scrub the outside of the zucchini with kosher salt to remove any of the waxy pesticides. Or use a peeler, but keep in mind, the skin is where most of the nutrients are.
Other people cook them MUCH longer like minutes instead of seconds, but like I said, you kill the nutrients the longer you cook them. You just want to soften them up a little & the otos aren't toothless babies. Make them work on the slice a little.
If you want to feed them spinach, julienne a thin slice for them. Giving them a whole leaf will probably create a larger mess.
They're TINY fish, so don't give them a portion that would require 10 otos to finish. That's why I would feed them a mini portion till I know they're eating the extra supplemented food.
If they finish, then I'll give them the more.
Oh yeah, those clothespins are good to use to hold the little pieces of veggies. Attach a string/fishing line to it for easier retrieval.