Oddly swimming Skirt Tetra

[FishInSpace]

One of my fish is swimming increasingly oddly, do you guys think he's sick (in which case what does he have and how worried for my other fish should I be) or is he just getting old?

I purchased some GloFish tetras 3 1/2 weeks ago (5 fish in a 10 gallon tank). One of the fish is clearly on the bottom of the pecking order. He (I'm fairly sure its a male) has always swam kind of oddly - occasionally swimming at an angle (somewhere between upright an a 45deg angle). If he were a submarine I would say he was having trouble keeping his ballasts equally filled. He looked healthy, and since this only happened occasionally, and always in the evenings, I assumed he was sleeping.

However I've just returned from a weekend away (used a weekend food pellet) and it looks like he's lost weight, and now its his front/back ballasts that are off, with his head way higher than his body. And he often drifts on the current rather than swimming like the other fish. I've taken a video of this that I will try to upload.

I've also noticed that he sometimes will lie down on the gravel - if I come over to investigate he will quickly join all the other fish in investigating the movement on the other side of the glass.

All the other fish look happy and healthy.

Any help/suggestions?

I'm working on prepping the bucket I use for cleaning the water into a makeshift tank - if he is sick then I doubt removing him will prevent the other fish from being exposed, but it might prevent any buildup of bacteria or ammonia in my tank if he does perish.

 

[FishInSpace]

file:///Users/amasoner/Pictures/Photos%20Library.photoslibrary/Thumbnails/2016/09/06/20160906-211205/5ekxK7HGS8KX5JA63FnVuA/thumb_IMG_8850_1024.jpg

Part of his apparent weight loss is an overbite - I've looked closely at pictures I took before this past weekend and I'm fairly certain his mouth looked like all the other fish's.

 

[FishInSpace]

oops, this is the correct link:
https://www.dropbox.com/s/bsh060dghqvb5cf/IMG_8850.m4v?dl=0

And it just occurred to me to mention that the fish I am speaking about is the green one.

Today he's in his own make-shift tank. I've been able to notice a few more things about his condition -
1) it looks like he has some dark spots around his soft dorsal fin, and maybe some on his spiny dorsal fin, though that could just be shadows. I've tried to take of picture.
2) he seems to be having trouble with his depth perception - I fed him this morning and he kept missing the food flakes.

I've been doing some internet searches and the closest symptoms I can find is "fish tuberculosis" (http://www.fishnet.org/sick-fish-chart.htm). Seems like a strange and exotic disease for a common skirt tetra - any thoughts?