Looking for some input here. All of a sudden, a few days ago, I noticed one fish does not seem to see. He doesn't interact with any of the other fish (or me). He either doesn't notice or doesn't care that his food is falling from the sky. He won't eat. There was even a piece of food floating right next to him that he was completely unfazed by. I have put my finger towards the glass and get no reaction whatsoever. Most of the fish will swim towards my hand outside the tank. He does react if I touch the glass; light touch, not tapping. He's not swimming around all that much, but when he does he seems to swim fine and doesn't crash into anything. He can dart when startled, but still seems to avoid obstacles. I'm not sure if that means he can see or what else he can sense in the tank. He's not moving a lot and seems to hang out in the largest spaces between decorations. Although, he doesn't swim at the top quarter to half of the tank which has no decorations in it. Most of them only come up about halfway from the bottom. Avoiding filter currents maybe? Anyway, not sure if this is something I should be concerned about or what. The fish is glow tetra in a fully cycled tank. Zero ammonia or nitrites. Nitrates are under 25. No problems with any other fish in the tank. Any thoughts would be appreciated.