Guppy-possible fin rot

[LizzieWallace]

Looking for advice please.
My daughter was recently given guppies as a gift.
We've never had fish before but we were recommended to set up the tank first, had the water tested (it was fine), get one guppy, test the water again then get the rest. We did all that. We use everything recommended and now have 5 guppies(all male). They had their first clean out yesterday (emptied approx 25% of tank, treated some fresh water then added it in slowly, making sure temp etc all stayed correct). We also used aqua salt which we hadn't originally used (used maintenance dose as on instructions. It was recommended by shop to use this).

We have one that's silvery with a spotty tail. It's nody seems to gradually be going darker. Yesterday we noticed it's tail was cut-it was like it had a slice through it. I've been keeping a close eye and its gradually becoming more ragged looking.
The fish still seems to be eating etc as normal.

Is there anything I can do to help the poor wee guy? Is there anything that can make him better/worse?
I've become quite attached to them all already.

Thank you.

[bunnygeek]

Fishkeeping initially can be tricky, and sadly pet shops often give out duff advice.

Are you able to answer this Q's:
-Fish tank size
-How long it has been set up so far
-Water chemistry readings for Ammonia, Nitrite and Nitrate (testing with a liquid kit is more accurate than with dip strips)