Lots of larger cities now use chloramine which does not evaporate if you leave it overnight. You have to filter it with a special filter, or use a water conditioner. I live in an area that has really good quality water, but it is treated with chloramine. So far I have yet to have a problem. I use a britta when they are babies which probably doesn't do a thing other than make me feel like I'm doing something. I have read conflicting reports of whether or not chloramine kills beneficial bacteria in the organic soils I use. I believe that it does kill some of them, but not at a level that is detrimental to my grow. I refuse to spend any money on a filter or water conditioners until I see with my own eyes a problem.

Chloramine - Wikipedia, the free encyclopedia
Scribble2010 Reviewed by Scribble2010 on . CHLORINE OVERDOSE I grow pot every year since I was 14, now I am 50, and I love to grow plant of all kinds. they all look nice. green and healthy. a lot of people that I know that grow ask me to look at there plants, and I tell them I am not a botanist, but I will look, and most of the problem originates from the same place, the tap water they are using. if you are using water strait out of the tap "STOP". first off it can causes lockout. it creates a very deceptive looking deficiency. purple in stems and Rating: 5