Guppy Tail Nipped

[spectrum]

I'm not actually sure if this is an emergency, but one of my female guppies (one with a big tail) is being harrassed by one of my 3 danios .
This danio in particular is slighly bigger than the other two, so i think that he thinks he is the boss. He chases the other danios and this guppy. I have noticed that the guppy has a ragged tail and is have a small amount of difficulty swimming.
I am worried, will she die? If so can i do anything?
I would be grateful for any help.
Thankyou.
Spectrum

 

[WILDER]

Danio's don't mix with guppys as there fin nippers.
If he struggling to swim in might not make it.
Can you issolate the guppy.

 

[spectrum]

Thanks for replying, I have a breeder net, but it isnt that big.
There is a small tank (about 20 litres) up in the shed that i could use but i dont have a heater i could use with it. I have some gravel that i replaced with sand in my tank a few months ago.
Thankyou.
Spectrum.

 

[WILDER]

I would rehome the guppys or danio's as they don't mix.
Hows the fish doing?

 

[spectrum]

It Died
Thankyou for your answers.

 

[WILDER]

Bless Him.
R.I.P.