Cephs are most sensitive to rapid changes in any parameter (salinity, pH, temperature), so if you have to alter something in the system, do it slowly. Do you have back-up water premade? I used to have a big plastic bin with 20-30 gallons of heated saltwater (I just had a heater dangling in the bin). So if there was ever an emergency, I was prepared to do a water change.
Was the new water the same temp. as the tank water?
Also, how big is your tank? The bigger the tank, the less impact new water has on the system.