You are trying to get chlorine to evaporate so it won't kill your beneficial bacteria etc in your medium. However most cities have choloromines in there water which will not evaporate out. Only a decent filter will pull them out.
canaguy27 Reviewed by canaguy27 on . WATER QUESTIONS? Hello, there is a theory that you should let tap water sit out for 24-48 hrs before using it, my question is if you let it sit out covered or sealed (I.E. old plastic milk containers with the cap on) is it doing any good, or does it need to sit uncovered? newbie thanks!;) Rating: 5