Guppy Died

[lucasluke]

So I came home to find my guppy dead. He had been laying on the bottom alot the day before and seemed like he couldn't swim well but he ate some and swam abit.
He was really thin. Also his gills were breathing really fast and you could see the pink when he was doing it.

Now I don't want other fish to do so does anyone have any ideas as to what could of happened? Some of my other male guppies seem to be getting really fat especially one of them.

Ammonia and nitrite are both 0 and I just did a 50% water change. Filter is mature. All fish have been in tank for about 3 weeks. Other inhabitants are corys, a dwarf gourami and neons.

 

[alihar]

Assuming your water parameters are all good and your tank is properly cycled and no other inhabitants are showing any signs of disease.
Has the fish been exposed to any kind of ammonia since being introduced to the tank?
It could be ammonia poisoning, maybe just a case of a weak fish or possibly a swim bladder problem.
Guppies are line bread for colour and finnage and so can naturally be weakened.
I've had a few guppies over the years pass as yours has and quite a few that have also looked a little fat compared to others.
Try feeding them a few cooked shelled peas once a week.
Peas act as a laxative and are good for the fishes digestion.
Alison