We have had a few fish in our commercial aquarium that have developed a thick area under the mouth before the gill line. The eyes would also swell and the fish died, no other fish got sick, is it worms? Maybe constipation?
1. I think it should be the bacteria and parasites that cause the eyes swell.
2. Treat the whole tank with Potassium Permanganate bath.
3. Then treat them antibiotic for one week. (Add salt)