Why has all 4 of my fish died?

I bought a fish tank for my nephew as well as 4 fish, can't think of the real names but a goldfish with huge inflated cheeks, a little chubby goldfish, and two tiny silver fish. I saved some water in some empty bottles so that it would be room temperature for when i came home ready to fill up the tank and put the fish in. Realising the water was a bit cold i put in some boiling water ( the fish were not in at this point) then some cold water and then some hot water from the tap. The water felt like it was at room temperature. Woke up this morning and all four fish were dead. Has the boiling water removed all the oxygen from the water? The tank has a 'water conditioner' with it which cleans the water but i don't think its anything to do with this. We were told not to feed the fish for the first day so we didn't. What has killed them?