i know this is an old thread but i wanted to add my 2 cents. instead of all this bickering and claims of "research" supporting one way or another, why dont' you all just do a water change and test to see how much chlorine is in the water afterwards without adding any conditioner? i'm sure everyone has chlorine tests being the experts that we all claim to be. i think doing a 10-15% water change is not even enough to warrant needing more conditioner as the chlorine gets so diluted is is basically non existent. even a 50% water change is going to yield half the chlorine concentration from your tap, so even if it does take 15 mins like some claim for the conditioner to work your fish are fine just the same. before anyone flames or adds a rebuttal, do the test and see for yourself.