I dont think this is common Silver $ which I am so used to seeing locally.
I think I see sharp teeth or is it my imagination?
It is difficult task to reach conclusion or verifying symptoms thru pic, let alone diseases, but I do see rather unusual red/brown streaks on its forhead from one side to possibly to the other side. Is there indentation or open wound above the mouth?
I am assuming implication of its Capillary System.
Provided that this fish was not exposed to UNIDEAL water condition for long time and its q/t water is in check, have you consider medicating with Kanamycin?
DO not mix with other med, just one type of med at a time. I would remove all previous active meds from water via small, frequent water changes and fresh carbon before trying any new med.
When fish displays such possible internal problems, showing blood vessels which is not usually visible, heavy metal based med is the last med, if at all, I would use to treat the fish.
One thing I do know is that when fish are exposed to extreme pH and possibly other factors, unideal to specific pH each specific fish has evolved to, it does affect the fish's gill functions which is respiratory and also excretory functions (ionic exchanges). When these functions are impaired due to inflammed/agitated gill filaments, their blood will be saturated with toxic waste products which can lead to diseases and eventually death.
I read enough threads from numerous forums that when fish displays red gill, all automatically turned to possible ectoparasite as culpit, not realizing that exposure to extreme pH would dispaly similar symtoms.
In such event, ectoparasitic med would do more harm than good and whole thing could ve been resolved by gradually correcting pH to suits specific fish's need.
I dont believe in exact pH either as nothing is in exact pH in nature. I am just saying try to accomodate your fish needs, avoiding extreme.
Please dont tell me fish can adapt. I know of their ability to adapt.
Ive done enough experiments over last forty year as hobbyist and over 20 yrs in industry with 20K+ systems with pretty much daily shipments of f/w & Ss/w fish.
Anyway, I said what I want to say hoping some will listen and minimize the suffering of their fish, not mine.
Is it possible to post some other angle shots? I am curious as to what species?
Unfortunately this is best I can do from pic.
Perhaps someone else sees something I dont see.
Hope all goes well!