R.I.P Little Fellow

I'm absolutely gutted.
I bought this fish 3 days ago, last night he seemed happy as anyone could be, swimming around, eating, following his little Oranga friend.
This morning, he was lying on his side in the tank with breathing difficulty.
I watched him all morning, hoping he would snap out of it.
His little fish friend was nudging him and even bringing him food, but he barely moved.
Ever so often he would swim a few inches, then sink back to the bottom of the tank.
I was going to take him back to the pet shop where we bought it after picking the kids up from school, to find out what was wrong with him and ask for a cure, sadly when we came back home he was already dead.
We took him back to the store, and the bloke just said: "ammonia poison, if you want I test your water"
Well, of course did I wanted it tested, after all I still have another fish in it and am not willing to put any more in there until I knew it was save.
Well, we went and got a water sample, he tested it and turns out the water is fine.
I did everything by the book and it seems to have worked.
So I ask him what the little fellow died off and he just said: "It was a dodgy fish, he would have died anyway!".....well thanks a bunch.
I was really upset that he died, because I loved his colour and everything, it's amazing how attached you can get to something that small after just a few days.
Well, I got an exchange and I named that one straight away.
This poor fellow died before I was able to decide on a name

Here one of the photos I took of him yesterday, while he was still full of life, playing with bubbles.
It's puzzling to me what went wrong with him.