A while back my crowntail started flicking, and while he never did show signs of ich or a typical cause for that action, I did find that one of the "water" plants I had been assured was really a water plant was not, and it had started to rot. I imagine that was fouling the water. At any rate, I took those out of his tank and got him actual water plants (some kind of *wort and a mossball), full water change, a little bettafix "JIC" and he stopped flicking.
Well, since that episode, he's pretty much healthy and happy and acts normal, but I noticed that he's very vision impaired on the side he kept flicking, and his eye on that side actually looks different. On his normal eye, the iris looks like it projects a bit, while on his blind side it looks almost sunken, though the outside shape of the eye looks correct. Is it possible that he damaged it with the flicking behaviour?