Clean water is the best way, so we don't recommend rainwater just in case! Most commonly used is tap water as it is cheap and readily available. Some prefer to stand the water for 24 hours or so to allow any chemicals to evaporate out of it, but generally speaking most people just get water fo the tap and use straight away. You can buy various bottled water but it would be more expensive and we're not sure if you would notice any difference.
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Clean water is the best way, so we don't recommend rainwater just in case! Most commonly used is tap water as it is cheap and readily available. Some prefer to stand the water for 24 hours or so to allow any chemicals to evaporate out of it, but generally speaking most people just get water fo the tap and use straight away. You can buy various bottled water but it would be more expensive and we're not sure if you would notice any difference.
Our friend Glenn at HomeBeerBrewery has a guide to making pond water beer, it doesn't seem to have done him any harm and makes for interesting watching!; http://www.youtube.com/watch?v=T9LCZXSh5Tw&feature=share&list=UUTMJpDs0Gls1eGxggOPf4OA