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.
It doesn't slow him down at all though!