Yeah it was a bit upsetting first time I had to euthanize a fish but when you think about it, it was better off for the Danio to simply not suffer.
I know it's a bit late now but first thing I saw was not a ballon-belly guppy but a very bloated gold longfin Danio
(like this: http
/www.google.co.uk/imgres?q=gold+longfin+danio&um=1&hl=en&client=firefox-a&hs=Vb6&sa=N&rls=org.mozilla:en-US
fficial&biw=1459&bih=924&tbm=isch&tbnid=Vqh2FwLMJ8AEsM:&imgrefurl=http
/bluegrassaquatics.com/catalog/product/view/id/1501/s/danio-longfin-zebra-gold-regular/category/7/&docid=0upU_YOy3PPYVM&imgurl=http
/bluegrassaquatics.com/media/catalog/product/cache/1/image/9df78eab33525d08d6e5fb8d27136e95/D/A/DANIO_LONGFIN_ZEBRA_GOLD_REGULAR.jpg&w=500&h=330&ei=ZPWWT5T3Oc7m8QPN5vz0CQ&zoom=1&iact=hc&vpx=686&vpy=157&dur=275&hovh=182&hovw=276&tx=163&ty=125&sig=114236487974604078664&page=1&tbnh=137&tbnw=183&start=0&ndsp=35&ved=1t:429,r:3,s:0,i:73 )
As FishFanatic04 said, having pets comes with a responsibility and whether you had a sick horse or a bloated fish, if putting it down quickly and quietly is the best thing you can do for it, then that's that. Next time won't be much easier (disease is almost inevitable for fish tanks at some point) since you get attached to the little beggers but try to think about it rationally