As all desease suggestions off sight with out seeing the specimen it's just a stab. I think it could be two things. My first assumtion is pop-eye. Do you notice an inlargement of the eye, is the skin of the animal clear or does it have small cloudy white spots? Although on the onset of pop-eye there usually isn't cloudiness involved, which is why I lean toward oodinium or marine velvet (amyloodinium ocellatum). If the fish is flicking and scratching against surfaces, rapidly breathing, very small spots peppering the body or the eye. This is the most infectious and deadly deseases. Swift treatment is a essential altough some strands of this desease are extremely resiliant to medications. Both diseases are treated with proprietary copper-based medication to kill the vulnerable free swimming stages of the parasitic lifecycle. Adding a UV filter will kill the free swimming parasites as well, but not for this fish unfortunatly. This treatment will cure both deseases, to help the pop-eye fresh water dip the fish prior to copper treatment just to make sure you heal what is wrong. If it is possible treat the fish in a quaranteen tank, I don't suggest adding copper to your main show tank. If you do not have the means maybe your local pet store has a fish hospital (quaranteen toank) you can use for the treatment. Thier are four types of deseases (mainly) parasidic, viral, bacterial, and fungal. Keep this in mind when accessing a desease and treatment, it helps to not be intimadated when theres only really four groups of problems. Also remember, although not talked about much in this hobby but fish can suffer from vost of the same physiological deseases as humans or other animals. Point, if a fish is ill it may not be one of the common problems, and sometimes there is no effective treatment.
