It wasn't eating anything and died shortly after isolating it. I do strongly suspect that it was internal parasites, and I feel really bad I wasn't able to help the little guy sooner. I've been watching the other fish and none of them look like they're suffering from the same thing so I think it might have just been the one.
My research led
me to look at prazipro and jungles parasite food as good options for getting rid of a parasite. If you are able to, quarantine the fish because if treatment doesn't work then the parasite will go looking for a new host if the original fish dies.
You could also treat the whole tank for parasites of you suspect more than one fish is sick.