hi there. I have in a 20 gallon: 50 endler fry, 3 panda tiny cory, 2 small apple snails. Never have problems with water quality and so on, so guessing its not over stocked, though once the fry show colour they get moved to different tanks and so on, if that helps
I've been told cory don't eat fry, though could eat their own. Basically if the fry is on the bottom and doesn't move when the cory comes over (which all healthy fry do, but ill and dead wont) then they are eaten. Which if its ill or dead isn't really a bad thing, so there you are. I have noticed in one tank with a colony of peppered cory and a colony of endlers that there were far fewer fry, though I lost a few adult endlers and 1 cory, so it could have been bad water, or the cory could have eaten the fry. I am betting on bad water, as when I fould the 3 dead fish, I stripped the tank, cleaned everything, replaced all the water and one filter pad, washing the other in tank water. Sinse I have moved the cory to a breeding tank, and I have loads of fry. So you could argue that the water change created a better fry environment, or moving the cory did. I will keep cory with fry in the future, its just at the moment they are all in breeding tanks on their own as I still haven't decided if female endlers will eat cory eggs or not, but I do feel safe keeping even the larger cory in tanks with fry. It keeps spare flake that the endlers miss eaten, lame and still born fry tidied, and keeps snail jelly at lower levels.
So, really, I would always have cory with my fry, except my cory fry, as being cory they are not threatened by larger cory coming over (as most of mine seem to sleep in cory piles) and therefore risk being eaten, where as guppy, endler, and other fish fry, will get out of the way of bigger fish (though they will eat together, when woken by a larger fish, they will dart off - I spent HOURS watching for the first few nights out of nervousness of adding such big cory to tanks with new born fry)
There you are anyway
These days I raise all fry seperate from parents so there is less food competition thus better nutrition, the cory for the clean up crew, and put the fry back in with parents when they can be sexed and recorded.
Sorry for the long reply, I am very "into" my fish, having just over 400 of them LOL!
Take care, and love your fish
(for adults, by the way, to answer how many for a 20 gallon, it depends on the length, if you have a tall, I would put 3-4, if you have a short, 4-6. Ideally I like my cory in groups no smaller then 6, and as its a fry tank and you will probably do water changes more often, it should be fine. I would use a sponge filter though, with fry, as a side note
If you ever want to chat or what not about fry, do let me know, its a favourite topic and my husband is so sick of hearing "what the fish did today"
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