Stroke/old age or injury?

[TLH]

One of my Cardinal Tetras has started swimming funny today.Its spine is curved to the left slightly and its belly seems to be slightly hollow.
It is swimming upright and seems to be holding it position in the water column fairly well but in sporadic bursts where it just thrashes its body to move,not just using its tail and fins.
They are all fairly old,maybe 3 years or more,I think,and this ones back is quite humped suggesting its age is advanced.

Could it have been injured by my shark or is it just its time to go or what?
What do you guys think?

 

[ReefRelics]

I'v seen that in Bettas, half the time it meens it hurt it's spined while trying to ich itself on the gravil or rocks you may have in the aquarium.. But what shark do you have??Its a possablity but you'r guppies could have also stressed the geezer out =3. However I find it hard that he/she is old o_o.... Dont tetras live up to 12 years???

 

[TLH]

The Rainbow Shark in my avatar and sig.The Guppies tend to stay at the top of the tank and the Cardinals inhabit the middle region so I don't think the Guppies had anything to do with it.

According to Shirlie Cardinal Tetras live about 4 years.

http://freshaquarium.about.com/cs/beginnerinfo/a/lifespan.htm

 

[TLH]

Well it's a week later and she is still here although her belly is a little sunken now.Being as she can't swim well she's not getting much in the way of grub.Give it another week and I may have to euthanize her.