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