That does look a bit like ich, I would treat with esha exit, it worked for me to remove ich and didn’t seem to upset any other fish in the process. I might also treat with esha2000 too in case the ich is caused by stress from other illness.
To be honest when you get a big school of fish there’s always a few that are a bit dodgy and don’t do well. Interesting that the tetras are quite nippy, I wonder if you could do with a bigger school to get the hierarchy amongst them a bit more balanced (once treated for ich). Same with pygmy cories - a large number helps to avoid them being singled out and calms them down as they’re used to big numbers. I know you want the whole a few fish in a big tank thing, but the bioload on these fish is tiny and I’ve found they do much better in bigger groups.
The other thing I’d do is to vacuum out all the existing mulm and then start again with a new layer of fresh dried leaves that will degrade over time. You can build up a good thick layer of leaves over a few weeks with a handful added every few days. I find this kind of leaf litter mulm much less problematic than new tank mulm, and it’s better structural habitat too, providing lots of hiding places for little cories. You can just add dried leaves to the top of the tank and wait for them to saturate and sink.
To be honest when you get a big school of fish there’s always a few that are a bit dodgy and don’t do well. Interesting that the tetras are quite nippy, I wonder if you could do with a bigger school to get the hierarchy amongst them a bit more balanced (once treated for ich). Same with pygmy cories - a large number helps to avoid them being singled out and calms them down as they’re used to big numbers. I know you want the whole a few fish in a big tank thing, but the bioload on these fish is tiny and I’ve found they do much better in bigger groups.
The other thing I’d do is to vacuum out all the existing mulm and then start again with a new layer of fresh dried leaves that will degrade over time. You can build up a good thick layer of leaves over a few weeks with a handful added every few days. I find this kind of leaf litter mulm much less problematic than new tank mulm, and it’s better structural habitat too, providing lots of hiding places for little cories. You can just add dried leaves to the top of the tank and wait for them to saturate and sink.