Most fish owners have made the mistake of forgetting the chlorine
remover, or not waiting long enough for the water to circulate fully
before adding our fish. The old idea that chlorinated water left out
overnight-24 hours will be safe for fish. That is no longer not true.
Only about 1/4 of the chlorine dissipates over each 24 hours. Norm
Meck has written a very clear and detailed article on the effects of
chlorine, poisonous plants and how to properly carry out water
changes. Here's the link to his full article on
http://www.richdeer3pondsupplies.com...246/age/930891

Chlorine burns their gills and tissues. Would you want to drink
something that burned going down? Neither do your fish.