They are the ones with an almost entirely black body, I think that's the brillant black varity?
I got 11 of them about 3 months ago. One has disappeared, I never found the body so it was eaten or decomposed under some wood I presume. One has spinal curvature and a somewhat bleached colouring, it's quite small too.
The two with bouyancy problems are otherwise healthy looking. They do it regardless of current direction, and as soon as they stop moving they sink - must be tiring, I get tired watching. Actually, in all seriousness, it detracts from the usual peacfull feeling I get watching the tank, just knowing the poor little guys are constantly struggling :-(
Is this species known for low genitic diversity, "Mah mother's mah sister" type issues?