Death In a New Plant Tank

My NEW plant tank is 4 weeks old.

Last Night while doing my weekly 25% water change all 3 of my black mollies 
died within 12 hours.  I just stick a hose over the side and carefully fill 
the tank.  I checked my PH,Chlorine,Ammonium,Hardness of H20.  All seemed 
fine, PH-7.4.

Can anyone guess on how I could have killed my mollies?  (They were 2 weeks 

Should i buy one of those fancy kits for transfering water from the tap?
Should i put my water in a barrel for a day to let the Chlroine evaporate?

John Coggeshall