I've been stick feeding my pink tail for more than a year now. Here's my post about 2 blind fish from the same source line:
https://www.reef2reef.com/threads/2-blind-fish.326906/
I believe it's the repeated exposure to multiple medications equalling a extended period of time. Or perhaps cyanide, which has been reported back on the scene more than you know. I had a niger go blind 25 years ago, and it was commonly known to be cyanide as the cause.
I believe it happens more than any of us know, had I not figured out it happened to my pink tail; I would have just thought the tusk went behind the rocks and died. They got fresh water dips and prazi pro in qt; there was no flukes or other observable issues in my tank. I fed the pinktail a kanaplex/focus mixed with his food to no avail; just in case it was bacterial.
I kept him in his home. a 210 fowlr. We have a routine; during feeding or when he wants to beg, he shows up in a spot that I have been able to stick feed him. He eats the best in the whole tank, nice chunks of seafood on a stick.