Welcome to TFF.
As other members have mentioned, the problem here is numbers of each species and insufficient tank space when it comes to all three. There is however more to it than just this.
When it comes to a community aquarium (one having more than one species of fish), there are significant issues to consider. This is why stocking calculators fail, almost always--they cannot possibly have all the data for each species of fish programmed, and without this knowledge there is a risk to the fish.
This sized tank would be ideal for just a group of 20 Celestial Danios. SF mentions this, and it is the best way to provide a suitable environment. This fish will (if healthy) be fairly active, interacting, but with this number no injury should result, and the aquarist is rewarded with interesting fish behaviours.
Alternatively, a group of the Glass Catfish would work space wise, with lots of plants and definitely floating plants for shade. SF suggests six as minimum, but this is only providing the bare minimum; a few more is a better plan, and will mean less stress likely, which means better health. There are some suitable tankmates, but note the caution in the SF profile.
Corydoras should be fine with the Glass Catfish, provided the substrate is sand. A group of around ten of the small/medium sized species. Not the danios as already noted.
A general comment to help understanding...fish kept under less than preferable conditions (preferable to the fish species) rarely show the problems this creates, until it is too late. Fish have inherent needs when it comes to numbers, other fish, water parameters, the aquascaping, etc, and it is only by providing as close to these as possible that we can assume the fish will be "OK." Like all animals, the will to survive and reproduce is strong, and fish will attempt to make do in order to achieve this aim. Providing the sort of environment they "expect" is the way to good health.
