Corydoras Aeneus-Male Is Ill!

Cories are very sociable creatures and don't seem to fare well unless in groups - 6+ and ideally more. (There are threads on this forum from last summer, when my cories succumbed one after the other, and I think the depleting numbers may have had at least part a role to play.)

What are your ammonia, nitrates and nitrite readings like? If there is any ammonia or nitrites, or the nitrates are high, that may be an additional possibility that accounts for what is going on, in which case a / more frequent water changes may help.

Presumably all the other fish are fine, as you don't mention them?

If you just got 2 new ones today and added them straight into the main tank, rather than putting them in a separate tank to quarantine them for a few weeks, then it's very likely that they may be "carrying" something which has affected your existing male catfish - and perhaps he's affected because his immune system isn't as strong as the others. The key course of action will be to keep a very close eye on all the fish. If there's any change or more are affected, let us know, and we'll advise further. Look out in particular for any signs of whitespot developing which is a common ailment when new fish are introduced; if it does, then you'll need to act quickly.