Is my Guppy is dying?

Question
Hello Michael.
So basically, I bought four guppies yesterday-two females and two males. I set up the tank as instructed and everything seemed fine. However, when I woke up this morning I noticed one of the male guppies looked like it lost a lot of color from its tail fin. I watched it for a while and the guppy seems to be breathing extremely slow and it frequently swims upside down. It doesn't move much. I quickly moved the guppy into a separate bowl  so it wouldn't be thrown around by the air bubbles and the tank filter. I plan on bringing the fish along with a sample of the tank water back to the pet store for them to check it out. I don't know if there is much to do right now but if you have any advice that would be much appreciated. Thank you.

Answer
Yael,
yes the fish is dying.
look close at the guppies as well as the sick one.
If you see a white band or patch anywhere from the dorsal fin back to the caudel fin thats the end of them.
it has no bearing on your water, this disease has been in the guppy population for years and is showing up more and more.
check the fish in your stores tank as well and if you see that, don;t buy them.