Ok doke....
Goldfish really do need to be in a proper filtered coldwater tank i'm afraid, one with plenty of room.
Their tank water should never be 100% changed, and should always be treated with dechlorinator (I use Tap Safe). Before I got my aquarium, I used to keep say about 20/30% of the old tank water to out in with the clean tank water.
The fresh water that you are putting in should also be the same temperature as the water you're talking out, not freezing cold straight from the tap.
However, despite doing all of the above, fish can and do die for the most unexpected reasons. However I really would invest in a proper coldwater tank for them, they're easily bought cheap(ish) from Ebay.
One last question- did the dead fish look any different? Was it looking bloated? Was it's scales sticking out, looking kinda like a pinecone?