I know from personal experience that my water will kill the fish. I was doing water changes on two tanks once and filled up one tank back up without adding dechlor yet. I started to syphon the other tank thinking that I would add the dechlor in a few minutes. Those few minutes turned into about 5 minutes before I turned around and saw all the fish at the bottom dying. I quickly added Prime and started swishing the water around in circles with my arm in the tank. I still lost 2 Redhooks, a Filament barb and 2 Datnoids. The other fish survived but I now know my water here, if untreated, kills fish quickly.