why is my guppy's tail very thin all of a sudden?

[raluca112233]

hello! ive noticed that one of my guppy's tail got very thin and i have no idea why. i check my tank every night and ever since ive had guppies, ive never seen this, she's been fine until now. what is the cause of this? is it serious? thank you in advance for your help! here she is:

 

[emeraldking]

The caudal fin is clamped but not thin. Most of the time it's stress caused by something which could be sickness, too much fluctuation in the water parameters or even caused by dominant fish. But there could also be other causes. Hard to tell from the info you've given.