Hi chin, using tap water depends on the chemistry of the water in your area and the plants you are growing, some plants like VFTs cannot take any nutrients in the soil while others like D. rotundifolia can take higher levels, also the higher the level of evaporation the quicker the minerals will build up in the soil so humidity has an effect on this too, and if your plants are outside rain can flush a great deal of the accumulating salts from the soil, or if you live in an area with a particularly low mineral content in the tap water then it will work just fine, if you call your city water supplier they should be able to provide you with a TDS (total disolved solids) reading, if I recall the cut off should be about 15. So as you can see there is a lot of factors to consider as to wether your water will work for your plants. Leaving the water out uncovered will allow the less stable chlorine compounds to evaporate but will not effect the mineral or organic salts.