my tetra is swimming vertically!!

[baz]

Hi
Came home this pm, to find one of my silver tipped tetra's vertical in the water, face upwards, i thought he'd died, but he swam away. He's very lethergic, and continues to swim vertically and then sinks to the bottom. I have tested the water, no problems there, does anyone have any ideas?? I's sure one of my other tetra's is doing the same!!
Thanks

 

[The-Wolf]

spunds like a swim bladder problem to me