After an episode of flicking brought on I believe by a rotting plant in his tank (frowns at Petsmart people who were happy to assure me it was a water plant, and at self who didn't go look it up before putting it in the tank) I'm pretty sure one of mine is blind or at least vision impaired in one eye. His eyes look different. The bad eye is almost completely black, where the other one looks normal. If it were a human eye, I'd say the iris is completely opened.
He doesn't notice food put on the bad side, and he is probably only about 50/50 when striking at food on his good side, whereas he seemed pretty accurate before the flicking.
My male betta Kraken is partially blind in one eye. When I look into his left eye, it is almost completely clouded over. He has trouble finding food and often doesn't notice me at his tank because he is left eye is facing me. Feeding him is a challenge because he misses his food but he's still very active.
I've always thought he may be vision impaired in one eye. If he swims with his left eye towards the outside of the tank, he doesn't see my finger, but once he turns around to the other side, he follows my finger perfectly. And sometimes he sees his food, and sometimes he doesnt. Its like he has to swim around his food so he can see it in his good eye. His eyes are also two different colors; one is red and the other is black. I cant check to see if its cloudy because he moves around too much, and usually he only has his good eye facing towards me.