Yeah I rarely feed them bloodworm (once every 3-4 weeks) but I have never fed my fish brineshrimp or daphnia so wondering how they would feel with that. Will post test results and a pic of the cory soon.
Ev
I have 21 of the larger cories, plus a colony of 30 odd pygmy cories, and trust me, they all go wild for frozen daphia and other frozen insects. Avoid the cheap foods like King British - you get what you pay for, when it comes to foods, and good nutrition combined with clean water are the keys to good health.
Omega one foods are incredibly expensive to get here in the UK for some reason, so I don't feed that. As a basic staple I use Bug Bites, which all the fish love and never seem to get bored of. I also use the FishScience catfish pellets, since Ian Fuller helped to develop them. The tablets they come in are huge though, so I cut them into smaller chunks with a stanley blade since a whole tablet is too much even for a big group of cories.
I also feed live microworms which are easy to culture, and good for both fry and the adults.
I get mixed packs of frozen foods, after checking what types of food they are of course! My current ones have mosquito larvae, daphnia, cylops, moina, and mini bloodworm, all of which are great for cories, as long as the bloodworm is a rarer treat rather than daily.
If you happen to have a bucket outside that has collected rainwater BTW, you can net out your own live mosquito larvae
Just check it first and make sure there are no dragon or damselfly nymphs in there before adding to your tank. If there are, you can still freeze them in ice cub trays to feed the fish at a later date, then no risk from dragon/damselfly nymphs.
Concerned about you getting readings for ammonia and nitrite - that shouldn't be happening if the tank is cycled and balanced, and is highly likely to be the cause of your cory's problem. That the others seem okay doesn't mean the water is okay, it just means that the one showing signs was weaker and the first to show signs and have it affect him more visibly first.
One common potential cause - how have you been cleaning your filter? Do you rinse the sponges under the tap?