Need Help for sick guppy

[GUPPY1966]

I have a female guppy and she looks like someone bent her in half.
In the same tank is 4 tetras and 1 other female guppy, 3 medium sized babies.
Its a 20 gallon tank.
Everyone else seem very happy and healthy.
She tries to swim and does for a bit, she eats, but she can't swim for long, like she is exhausted.

Any ideas???

 

[librarygirl]

So her spine is curved? Can you post a pic possibly? How long has the tank been set up? Have you tested for ammonia, nitrite, nitrate? Is it cycled?