One of my male rosy barbs seem to have a difficulty to either tell up from down, or to maintain his body position. When the lights are on, it looks a bit goofier than others, but nothing too drastic, except during feeding when it will go in all random body positions to get to food, upside down, sideways, head down, you name it. But when the lights are off, he seems to lose the sense of direction entirely, orienting itself towards the light comming from the room (1:15 in the video). It's like he can't actually feel gravity, and only orients itself based on visual cues, like an astronaut in 0 g. It eats and is curious, active. His colour is less vibrant than other 2 males, and he was like that ever since I got them half a year ago. He had a weird pimple on his head a month ago, but it went away in a couple of days.
Tank is 30 gallon, water parameters are 0, 0, <10, pH 7.2, temperature 76 F/ 24 C.