How long should tap water be left to sit before a water change?
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Expert Answer
If you use tap water for fish, it is best to draw some out 3 days in advance and leave it in the sun for 2-3 days before doing a water change. The reason is that tap water contains a lot of chlorine, which is harmful to fish and can even be fatal. Sunlight helps the chlorine evaporate, and after 2-3 days of airing most of it is gone and the water is safe for fish.