With the Asians, I have a 26 -27inch silver aro, he has been feed the best pellets his whole life, as well as feeders, general crappy goldies/red caps/moors. etc.. just what ever was around. he has perfect eyes, and not to mention scales which are fantastic
I've had him for roughly 6/7 years. I think I am safe to say he won't get dropeye.
From what I've read and shared with other aro keepers in Australia/Sydney, they say that a number of things diet, environment etc...
The most common thing from what I hear is, having no sand/rock/gravel or light coloured gravel/sand making shadow from the lights above, or no gravel so the reflection of the aro is always there, since the aro is always on the move, it's looking down at itself thinking it's other fish and years of looking down creates drop eye, also allot of movement below can trigger it. I have stayed clear of over populating my 6x6x6 aquarium. I have a few gars and some plecos, all of which are dark with black rock on the bottom cutting out the shaddow and alot of movement below him. but he is the largest and most active in the tank anyway!
I have had a 18inch (asian) aro develop drop eye over a 1 year period also, so I think it may have to do with environment. That is a personal opinion though it can not be backed up, I enjoy my aros a lot, but I had to sell one of them, which was about 13inch long, and with in 1 year it got drop eye, the guy who I sold it to didnt have gravel..
I wish I knew how to 100 percent prevent it, but I have a feeling it's generic and also it's environment. But thats just my personal opinion.
Cheers!