Just to elaborate, what triggers having to do a water change during an emergency quarantine? I am monitoring the ammonia level in the hospital tank and treating for ich. I am doing a 75 gallon hospital tank and have 20 gallons of water ready, already mixed just in case. I also read something when researching that said when doing a water change and treating for copper, do not just treat enough copper for the water you are changing, but completely treat the entire tank again. That just does not even sound right, so that is why I am back here!