Octavia,
How long has this tank been running, were the params off before you did this water change, was it to correct a probem? Ammonia or Nitrate buildup? Were the inhabitants showing signs of stress before the water was added? I find it a little hard to believe that roughly a 10% water change would kill off all your livestock in an established aquarium, I could see some signs of distress if the replacement water was not close to the tank water, but all to die,hmmm was the water contaminated with anything?