Fish woes - Part 1

My cutest little fish died today :(

I bought an aquarium 4 weeks back, and it was the lovliest thing ever - 4 tiny red mollies. Loved to watch them play with each other, and this one was the youngest of them - very naughty and very adorable.

I saw something wrong with it yesterday.. and couldnt figure out what - it was jus lying in a corner, was not eating, rather was not able to :( but I just couldnt think something could be so wrong..

I was sitting next to the aquarium today morning, and saw it struggle, as if gasping for breath. It circled around once and sank, and then again and again.... and then when it sank.. it never rose again.. it went right in front of my eyes :(
(in the pic : my lil fishy that died, few hrs before)