Neon tetra - darker, more aggressive

[fisherjean]

I'm going to try to get an image of him, but one of my neons has been quite the chaser in the last week. We've had some water parameter fluctuations, from my not knowing about GH and KH when using only RO water. Things are improving, but in the meantime...

One neon is darker, very dark blue - like black dust over blue - both on the belly and back and around the dorsal and tail fin, and he's bothering the other neons.

Velvet? Something else?

 

[OrangeCones]

I've never seen a fish suffering from velvet being the aggressor. Usually they will be picked on by the healthy fish.

Does the fish have small spots that are white/grey/light yellow?

 

[fisherjean]

Man, it's really hard to tell. It just looks like a black dust on him. I've gotten a couple of terribly unhelpful shots of him I'm afraid. He's just too darty.

As far as schooling fish go, should they be together all the time? At what point is wandering suspect?