Side discussion: I won't be introducing anything new for a while, but do you have any suggestions for schooling fish (to replace the danios/WCMMs) that would be peaceful and not bother the cories?
Oh yes, there are lots of options! So so many that you're spoiled for choice. Especially given the following things you said...
I was looking at emerald rasboras before the cory/oto saga, and descriptions say they're peaceful, but also that they are playful/active swimmers so I'm not sure if they would actually be better.
They definitely would be better! There are the emerald rasbora, and their close relation, the celestial pearl danio, or you could consider fish like ember tetra, chili rasbora, green neon tetra, kubatoi rasbora... there are so many options! Look at what's termed "nano fish", and "nano tanks". A 20g long gives you a huge amount of options, and those fish are active and swim around, but won't outcompete or scare the pygmy cories the way a group of zebra danios and WCMMs do.
The pygmies don't mind active swimmers or shoaling fish like the above mentioned, they enjoy the activity in fact! They'll often join in the group when they're in the mood, and when they pop up to the surface for a gasp of air. They're shy and more withdrawn if the upper level fish are much larger, more aggressive, or are at risk if the upper layer fish are voracious eaters, like the zebra danios, so they get outcompeted for food.
But you can still have peaceful, colourful, active and fun to watch large groups of small fish like the ones above, and in a big group like a school of 20 pygmies, 6 otos and a group of 12 ember tetra and 12 celestial pearl danios for example, you could manage that in a 20g long once it's well established, if it's also well filtered and well planted, and you do the weekly maintenance, and make sure you have the right tiny foods for these fish! It's very do-able.
Some frozen foods like cyclops and daphnia, whether live cultured or frozen, are especially good, because they float like a cloud in the water column, allowing all the fish to get a chance to eat. They're often used for feeding fry for the same reason, and for their small size. Since pygmies and most of the species mentioned above have tiny mouths, the food needs to be tiny. I've used a pestle and mortar to crush dried food into smaller pieces, until a friend recommended getting a pepper mill, now I can just grind the Bug Bites microgranules even smaller straight into the tank.
I'm also interested if there's any danio/rasbora/tetra type fish that's a little smaller (like <1 in. rather than 1.5-2 in.)? Seeing the cories schooling together has made me realize that having a big school of tiny fish is more fun than the other way around.
I'm so happy to read this! I completely agree. I started off with seven pygmies, because that was all the store could get at the time, and I planned to buy more when they were available, to bump their numbers up to 12. Well, I didn't need to, because they surprised me by breeding in the tank, I was cleaning one day and saw two tiny baby pygmy corydoras hiding in the plant! Now they breed and the fry raise themselves in the tank, and it's even more lovely when you see a group of 20 or more swimming around together!
Since these fish live in huge numbers in the wild, you see why we see different behaviours when they're in a larger tank (yours is a lovely size for keeping large numbers of these tiny, but active and beautiful fish), and in larger numbers of conspecifics.
I'd recommend rehoming/returning the zebra danios most urgently, because of the greedy voracious eating thing, but then take your time choosing what you'd like instead. Watch videos of the fish you're interested in, and go to stores to see them in person! Pictures and videos don't always do fish justice. But they can also be pale and stressed in store tanks, so seeing both how they look in someone's established tank schooling around, and seeing them in person can help you decide. Just resist the urge to impulse buy fish when you visit!
Check out this video - this youtuber is really good and knowledgeable, a fish breeder and trader, and in this video, she'd been bought a huge school of Corydoras pygmaeus and Corydoras hastatus, and even she was amazed at how lovely it was to see such a large group of these two dwarf corydoras were feeling secure together in a huge group!
Skip to the 2::45 mark to see them!
), I'm not sure if it found the food. I did notice that the other fish were also eating the food at night, but it might just be because I had a lamp on in my room and they were still awake when I put it in. Now that we've established that the food works, how often should I be giving it? My gut says to keep giving some every night so the other oto can learn, but I'm afraid to overfeed. I definitely saw the platys eating it but they don't seem rounder in the morning so maybe the platys stopped when the lights went out, and the otos did get most of it.
