Cant you feed them pellets that sink to the bottom of the tank
There are commercial and personally cultured foods you can supply for them - but as said above, these are wild fish that have been scooped out of their natural habitat, stored in overcrowded tanks and sometimes spend days in bags of water with no food being shipped across the world.
So they haven't been raised in a tank, eating algae wafers. It can take a long time for them to even recognise some commercial foods as being food - it's not the same as they'd eat in nature, and what's in the pellets you have in mind? Does the food you're trying to coax them to eat the right food for their nutritional needs?
Repashy soilent green is one food that seems to be doing really well for people that keep grazers like otos, and herbivorous plecs. But because it can take time for them to settle in, recover from the being caught/warehoused/shipped/displayed in a fish store/bought and taken home to your brand new tank, that doesn't have the natural algae and organisms that these fish naturally graze on, you're much more likely to lose the fish. That's all we're saying. They're a delicate fish that need stable conditions and an established tank.
There is a lot of misinformation out there, and bad fish store advice- because the staff either don't know, or they want to sell you the tank, decor, fish, and a load of products for you to add to try to speed up the process. Because naturally, most people when they get an aquarium, they want to fill it, get it running, add the fish, and have it all set up, beautiful, and "finished" in a week or two.
But that's a super common beginner mistake, and be wary of people giving you advice about tanks if they're trying to sell you something at the same time. Of course they're not going to say "your tank is brand new and not cycled yet, don't buy these otocinclus I have right here and am trying to sell right now - just get plants and some tetras for now, and wait six months for otos."
They're not going to say that and talk themselves out of a sale, you know? We're giving you this advice based on the welfare of the fish, and to try to help you avoid the common beginner mistakes that end up causing them problems in their aquarium before it's even really got going. We all gather and share our hobby and learn from each other.
If you're doing a planted cycle and adding schools gradually, as @Essjay recommended and many of us agree with, then you'd want to add the otos last anyway. You need to give each group at least a couple of weeks for the nitrifying bacterial colonies and plants adjust to handling the bioload of the new group of fish you just added, and to monitor them for at least 2-4 weeks, testing the water fairly often, to make sure they're healthy and the tank is getting well established, before adding the next group, etc. Ideally you'd also quarantine each new group for 2-4 weeks in a separate QT tank before adding them to your main display tank too... the last thing you want is to have the first group do well, add another group, then find they've introduced a disease or parasite or something to your main tank and now you're previously healthy fish.
Sorry for the long response! Trying to help, but not good at being concise, even at the best of times! Lots to learn in this hobby, but it's really rewarding when it winds up working out.
Seriously Fish is a really reliable, accurate dataset about most any species of fish you're likely to find in the hobby. Always worth googling "Seriously Fish otocinclus" or "seriouslyfish ember tetra", and reading their profiles on the fishe's needs, diet, suitability for your tank size and stocking etc. With so much misinformation out there, it's good to have a reliable site to get a good overview of any fish you're considering adding to your tank.
Here's their profile on one of the most common otocinclus species, but same advice for any otocinclus species, except zebra otos since they're much larger.
Otocinclus macrospilus – Oto — Seriously Fish
And here's their profile on Ember tetra.
Hyphessobrycon amandae – Ember Tetra — Seriously Fish
And a really good article about otos and the care they need before buying them.
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Keeping Otocinclus catfish in the aquarium
Keeping Otocinclus catfish in the aquarium
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