natural deformity, maybe. I have several danios with gross deformity.
Levels in my tank are immaculate.
Stress, possible, as they have very little swimming room these days due to plant overcrowding. (trying to grow out)
cute - it is.. especially when they swim. Poor guys.
I'm not sure if that other guy is still around. I'll have to peak around. It also causes rapid gill movement. Other fish are not experiencing the same symptoms, but it does seem to be spreading slowly.