Strange Tetra Behaviour

[racegoer]

Hi,
I have a well established AR-850 [160 litres] with a variety of small fish, 2 Zebra Danio, 5 Silvertip Tetra, 8 Black Neon Tetra, 6 Cherry Barbs, 2 Peppered Corys and 5 Diamond Head Tetra.
For the past week or so 2 of the DH Tetra have been behaving in a strange way, at first I thought they were 'displaying' like 2 of the Silvertips do--beautifully--- but now I'm not so sure.
They are almost vertical and moving very fast 'on the spot' Sometimes they do do it next to each other but not always, I have added a short mov to give an idea and also a picture of the normal ones all trying to eat the Corys wafer !!!!
Any suggestions, all parameters spot on.

 

[drobbyb]

Looks like they are swimming against a current.

 

[racegoer]

I don't think so, but I know what you mean. I have a Fluval 4 in the tank and it's directed along the top, the DHeads do this vertical swimming wherever they are in the tank and everyone else is very relaxed most of the time. It's quite tireing watching them!!!!!