I have go on the 'odds.' Odds say that the greatest probability is that you fish has been slowly loosing its eyesight over a long time. It is just that it has gotten to the point where it has become noticeable.
Sight loss is gradual and I would suspect that you would not have noticed it creeping along until something brought it to your attention.
I can only go on the greatest probability here, because I don't know your system nor what you do. So, we can start there if you want to begin again. Please take your time and thoroughly answer these questions:
How old is your tank? When did it originally cycle?
What kind of system do you have (tank volume, dimensions, bio-filtration method, the equipment you use, any carbon or other chemical filtration, kind of substrate, etc.).
List all specimens & sizes in the tank (fish, inverts, corals, clams, snails, crabs, shrimp, etc.).
If you have a refugium and/or sump, what is living in it?
Do you use a quarantine tank and procedure?
Foods you use and feeding schedules.
You say you've had this fish for four years. Right?
Do you use any vitamins? Fat additives? Any elemental or other additives? Please list all.
Chemistries ââ"šÂ¬Ã¢â‚¬Å“ please give actual numbers (Ammonia, Nitrite, Nitrate, Phosphate, Silica, Ca, Alk., and any others you have)
Water parameters ââ"šÂ¬Ã¢â‚¬Å“ please give actual numbers (pH and your pH range, salinity or sp. gr. & range, temperature range)
Do you see any of the following in your system: hair algae; micro algae; cyanobacteria growths; dinoflagellate (zooxanthellae) growths; brown algae; diatom growth; slimes; colored patches on rock or substrate; etc.?
Water changes (how much and how often). Kind of artificial salt youââ"šÂ¬Ã¢"žÂ¢re using. Kind of source water you are using? Have you analyzed your source water for the above chemistries? When did you do these tests last? What is the TDS of your source water?
List what you added or taken out of your aquarium system (living, decorations, and equipment) during the past 6 weeks.
I think the above will give us a good foundation and starting ground.
