More than likely either your fish is suffering from a disease, Or your fish and system are infested with a parasite.You say its been on off and for around 6 months? sounds like ich to me. Velvet kills and does so very quickly. Ick is more Survivable and as long as the fish isnt stressed and is otherwise healthy they have the possibility of not showing symptoms, Stress from new additions or drops in water quality will usually cause an outbreak in this case.cloudy eye makes me think Flukes is also a possiblity. Though it could be something else. Could freshwater dip him in RO water with the same PH and temp and see if anything falls off to eliminate that possiblity, in that case id just do two treatments of prazipro.
In your case since you dont have a QT set up(.i highly recommend setting up one, a 10 Gallon tank and a cheap power filter and along with some ammo lock and seeded filtration media will work fine, Can be bought for under $50 and will save you alot more by preventing fish death's and medication, along with time,effort,and frustration.) i'd freshwater dip him to eliminate flukes as a possiblity, Because if it is flukes your lucky and that can be treated in the Display tank with no harm to anything but featherdusters(allegedly, it killed nothing in my tank, absolutely nothing, as far as beneficial organisms go anyway.) if not flukes it would probably be best to get him into a seperate QT/Hospital tank to better diagnose/Treat him.
If its ICH like i mentioned before you will have to take all fish out (Going "Fallow") and Treat them in a seperate QT tank while Leaving the Display without ich for at least 72 days. Treatments for ich are Tank transfer method(TTM) 3-4 weeks of Copper Treatment(I recommend cupramine.) And hyposalinty Treatment( The most difficult to Use as its hard to maintain Salinity at proper levels, and some Strains of ich have been reported to be resistant to this method.) More information on these methods can be found in the Stickies Located in this section of the forum.
Could be something more along the lines of a bacterial infection which im not 100% familiar with so i dont feel comfortable giving advice about that except that theres quite a few stickies on this forum regarding diagnosis, Treatment, and General information on fish disease's/Parasites that i suggest you give a read.
good luck!
