Whats Wrong With My Guppy?!

I noticed that my guppy was swimming at the top of the tank the past two days unlike the others swimming around happily. Today I realized his back end was sinking till he was vertical and then would swim a little to straighten out. I know he is not doing well because he is not eating either, my only concern is could this affect my other guppy’s? What should I do with this guppy? I have separated him just incase but I don’t completely know what's wrong and I am unsure what to do. Pictures are attached to show what is happening.

 

It’s possible he was previously injured or that he has internal parasites. Look for white stringy poo or an inflamed anus, or protruding worms

 

Crimson_687
I did some research and I didnt see any of that, he winded up passing away, when I brought him into the store he looked like it was his time and then he started swimming again. They told me to try a salt dip but when I got in the car, he took his last breath. I'm just hoping my two other guppies are fine. Looks like they are getting along with the new guy

 

Crimson_687
I did some research and I didnt see any of that, he winded up passing away, when I brought him into the store he looked like it was his time and then he started swimming again. They told me to try a salt dip but when I got in the car, he took his last breath. I'm just hoping my two other guppies are fine. Looks like they are getting along with the new guy

Sometimes they just don’t make it always quarantine new fish so that your existing fish don’t get sick

 

I did! Good thing cause another one died, now I have five of them in their tank with a filter and a bubbler. They’re all happy!

 

Most of the time the majority of probs with guppies dying is overfeeding, have you been feeding them much? I see too much food bits.
It's possible that it gave it digestive issues which give them trouble swimming. I once had a sick guppy that I took out and kept in a small cup like yours...it died as well, biggest mistake ever, that cup is way too small and when you put him in I hope you added his original tank water.
If I ever see any problems with swimming or tired looking guppies then its a sign I've overfed them, I just stop feeding them for a bit and they mostly return back to normal.