Thanks everyone, that's really helpful and makes sense
I'll try and get the dewormer soon. Speaking of which, do I get the one for dogs and then if that doesn't work the one from Petbarn? Or just the one from petbarn?
However, what if it isn't worms? What if the pygmy is just really skinny? (it's not getting much fatter, no matter how much I feed it).
I haven't seen anyone in either tank do a stringy white poop, and the fish in the big tank also don't seem to be skinny (or any skinnier than usual).
Another unexpected development- the pepper sleeps floating. I think it's swim bladder is broken. It can still snuffle along the bottom, but only if it's putting a small amount of energy into staying on the floor. When it's resting, it floats up, tail first, maybe a centimetre or two, then seems to catch itself and go back down again. Interesting.
Sorry for being rather difficult and making this go around in circles, I really am trying.
Video or at least photos please.
Sounds as though the peppered cory is suffering and at the point where euthanasia would be kinder. But without being able to see anything of the fish and a vague description, impossible to know.