When using tap water its a good rule of thumb to let it sit for 12 - 24 hours to let the chlorine, etc. evaporate. the same thing happens when you bubble your water, ALSO you add more oxygen to the water than it already has. Most tap water is not saturated with o2, just enough to make it water, the air bubbles added to the water do contain more o2 than the water originally contained. if you are using a reservior then you need to add the bubbling action otherwise the water becomes stagnant, therefore you do add o2 to the water, again negating the comments that it doesnt. otherwise you would see aquariums without those little bubbles coming up, etc.