Your tank and your living room looks very nice and clean. What happened to the fish once you changed the water? Were they gasping for air? It seems a little strange that co2 in the well would kill your fish.
It was very strange, but the only thing that seemed to make sense (more discussion here Fish die-off after water change/plant moves - anyone ever had this?
Put a cup of tap (well) water on the counter, after 12 hours the Ph had risen substantially.
Some fish were hanging at the top - and the ones that died were hanging motionless, like they were stunned, swimming/moving a bit before becoming still again.