Inactive Guppy

[lindzee]

I have a 10 gallon tank and bought 3 male guppies 4 days ago and they all seemed fine. For the first 2 days they all swam around together and looked like they were having fun. On the third night one seemed to have a problem. He had a long reddish/brown string of poop (?) stuck to him for most of the night and into the next morning. It finally fell off and he was back swimming with the other 2 like normal. After a couple hours he was off sitting alone in the corner of the tank by the heater and he's been there ever since. He stays within a couple inches of the gravel or right down on the gravel, hardly swimming anywhere. He's constantly opening and closing his mouth and hasn't eaten anything in a little while. I've been feeding them flakes, tried feeding him a pea but he wasn't interested. Sometimes the other 2 guppies come swim beside him and try to get him to swim with them but after no luck they give up and swim away. 
 
Any ideas what could be wrong or what I could do? 

 

[Deepatlantis]

You may be over feeding? The poop sounds the right colour, just too long. It's a good idea to feed every other day the first week. Also, did you cycle the tank? If not, you need to change about half the water immediately and read up on cycling a tank. Did you use any tank starting chemicals, water treatments, etc?
He does look very bloated/fat in that photo.

 

[lindzee]

Ive fed them only a small amount twice a day, I didn't think it was too much. The other 2 would always seem to eat more than this one but they're still fine. The tank hasn't been cycled for long and I have been using a water conditioner and a fast acting bacteria starter. Water tests have turned out ok, with some nitrates being produced.
Last night he started moving around some more but now just sits under the surface of the water. He doesn't look as fat though today.