WHY DO FISH GET GLAZED EYES ??

Hello everyone:

Every now and again, I see a fish (for example a Bala shark or
Gourami) with glazed eyes either in a pet store or in my own tank.

This happened to me one time ... I purchased this Bala shark however
after a week, developed glazed eyes and eventually died even though
the water quality was well within acceptable limits.

Does anyone know HOW this comes to pass? Is this caused by a parasite
or something? What about cures .... is there a way to cure this
infliction??

I'm not sure if this little bit of information helps but I picked out
this Bala amongst hundreds in a tank ... and in this tank, perhaps 4
or 5 of them already had glazed eyes.

Any feedback would be appreciated.

Thanks All !!

Jim.