4 angel fish loosing site?

My son has 4 angel fish that seem to have gone blind or maybe just have developed very poor site.
He bought them as juveniles where their body was about the size of a quarter, and all 4 are now about 4 inches in diameter.
Other than their newly developed site issue, they look healthy. Their eyes are clear with no shrinking of pupils or cloudy whiteness in them.
Before when my son just got near to feed them, they could obviously see him and would quickly congregate to the top.
Now, they don't see him, and do nothing. I told him to tap on the glass a few times before feeding and they are now trained to go to the top for feeding. Also before, they use to see their food and chase it. Now they just to to the top and start sucking until food happens to go in their mouths.
They seem to be eating well and like I said earlier they look quite healthy. They just have all gone blind or developed some kind of eye issue.
When I googled "angel fish going blind", there seems to be lots of this going on. It almost sounds like a genetic issue with modern angel fish.
Maybe too much inbreeding causing this odd vision problem when they reach a certain age? Just a hunch. Has anyone encountered this as well?

BTW, 3 of them are koi and one is a Philippine angel fish.

I've checked the temperature and water parameters and all within specs. In fact, my son has a 2 horse face loaches, a mating pair of corydoras laying eggs all over the place, and 2 dwarf plecos one normal color and the other albino. All are doing fine. So I'm scratching my head wondering what the heck is going on. Hence the hunch of genetic defect with the angels. Like I said, all 4 use to see us and the food and they use to chase the food down greedily. Now they just bring out their white canes and start sluping blindly at the surface. And we tap the glass to let them know it's feeding time or they don't even know it's time to eat. So it's a total mystery to me and my son. I guess the good news is they are still healthy and eating.