Found guppy hiding at the bottom. Seems to be unable to use tail fin?

[OskarOskar]

Hello i have sex guppies. I bought them last week. I found a female hiding at the bottom of the tank, the other fish visited her at times and looked at her tail fin and as I watched her she seemed unable to swim with her tail fin? So I have put her in a breeder tank because I thought it might help her a little to be close to the top with better water conditions? But like I said she can’t swim with her tail. As you can see from the photo the tail is pointing downwards. Is there anything I can do?

 

[Colin_T]

Can you post a 1 minute video of the fish trying to swim?
You can upload videos to YouTube, then copy & paste the link here.
If you use a mobile phone to film the fish, hold the phone horizontally (landscape mode) so the footage fills the entire screen and doesn't have black bars on either end.

How long has the aquarium been set up?
What other fishes are in it besides the guppies?

Is the fish eating normally?
What does its poop look like?

What is the ammonia, nitrite, nitrate and pH of the water?
What sort of filter is in/ on the aquarium?
How often and how do you clean the filter?

How often do you do water changes and how much do you change?
Do you gravel clean the substrate when you do a water change?
Do you dechlorinate the new water before adding it to the aquarium?