Dead fish! Can someone explain???

[twoplankskier]

Okay so I originally had a 10gallon FW tank with a filter that sits inside. (Bottom half is sponge top half is carbon, air bubbles rise through and suck water though the filter...so I am told) I had 2 fake plants, a large log and a small treasure chest. My ph level was 7.6, I had added the tap water conditioner and the fish stress coat and the water clearer. The tank had been cycled for 7 days before adding the fish. I floated the fish for 30 minutes, then put a cup of my water into their bag for another 30 minutes, then netted them out into the tank. I had done partial water changes and siphoned out the gravel a couple times, I also had an air stone bubbling thing powered by an air pump. The goldfish died within an hour of each other in 8 days.

I cleaned out the tank very thoroughly. Washed the gravel through a sieve, scrubbed the ornaments and plants, tossed out the old water, got new filter cartridges, a heater, and a temp gauge. I cycled it, and let it heat for 4 days before adding the new fish. When I purchased the new fish I also purchased an under gravel filter. I removed everything out of the tank into clean buckets and installed the filter, then proceeded to put everything back into the tank, including the cycled water, which I had kept in a clean bucket. At the pet store I had also bought a ph lowering kit. I added the prescribed amount and waited an hour before testing. It read a happy 7.2. The fish had been floating in the bucket for 45 minutes, I then added one cup of my water to theirs, waited another 45 minutes, then dished them into my tank with my net. Everything was going smoothly the under gravel filter was working like a dream, the air stone was bubbling the other filter was working as well, about 6 hours later 3 fish were dead.

I have no idea why this is happening. ANY help would be appreciated.

Cheers,
twoplankskier