Have you tried pulling the canister down and checking the for broken parts? Or something caught in it maybe?
8 yrs isnt that old.

404 on 125gal, personally i dont think thats enough. I have a 405 (+ an internal) on my 90gal and am looking at adding another canister to it.

Depends what wrong with your fish's eye. It may have actually done damage to it (ran into something for example) and could now be blind, hence why the eye is white.

Have a very close look at the eye, even to the point of catching the fish and checking it out of water (not for too long though obviously). You need to work out if its caused by an injury or a disease before you can determine what treatment would be required.

Do you have a smaller tank you could move the cloudy eye fish into to treat, rather than treating a 125gal tank each time?

I would be tempted to treat with an antibiotic, but that will mess with the bacteria in your tank, so treating the fish in a hospital tank would be a more ideal situation

As i said with my tank i have a Fluval 405 (+internal) and i am looking at adding another, definately wont be a Fluval though. Probably a cheap SunSun

I run a SunSun on my 75gal tank, it is quieter and does a far better job than the Fluval and is one third the cost!!