30 Gallon Tank Rosy barb balance/coordination problem

[V1K]

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.

 

[A201]

The swim bladder as well as other organs can be affected by virus, parasites & bacteria.
The noted pimple might have been an indicator or a virus. IMO, the instability issue probably isn't treatable & likely is not readily transmittable.
You sure have provided a beautiful environment for your Barbs.

 

[V1K]

Today his condition has worsened significantly. I saw it hiding close to the bottom of the tank and barely moving. When bothered, he swam away in erratic movements. I considered euthanasia, but eventually I just took him out and put him in a bucket with a mini filter, some main tank filter water for BB, and water lettuce for cover. I have low hopes of him surviving.

 

[V1K]

The barb is still alive and, surprisingly, still eating, but his movement are very erratic. It would sit on the bottom and move slowly in a proper position, then suddenly go into this frantic spinning and flipping upside down etc, and then calm down again. I'm reading that this kind of thing might be a brain infection, or a whirling disease. The latter is transmitted from tubifex, and I do have freeze-dried tubifex as one of my foods, so I guess I'll be throwing them out although it might be to late as I was feeding all fish with it for the last half year. I wonder, if the cause of this barb's disease could be the same as the deaths of 2 of hillstream loaches. The loaches did not whirl nor flipped and didn't have these convulsive episodes, but they did seem to lose bodily control, unable to crawl the glass or run away from nuisance...

 

[V1K]

I decided to euthanize. He wasn't dying but it wasn't getting better either. I didn't get much answers here, but what I managed to find out myself seems to point to either a variety of brain infections/parasites, none with much hope for recovery.