According to the website you add it to your tank as it will not hurt the fish. You can add it after you already added your water with the proper silinity. What does it say on the bottle?
http://www.reefcentral.com/forums/sh....php?t=2195588
Read that on how to setup a QT tank. If its an emergency QT that is not runnkng 24/7 fill it with water from your main Display Tank (DT for short). If it's a QT tank that is running all the time it needs to be cycled.