Tap water is entirely dependant on where you live, so is rain water and well water. Water is not just water!! most companies like poland spring RO there water to get that good taste. Tap isnt bad, however u need to let the chlorine evaporate like noted above. I use well water, and its got loads of minerals in it i need to account for when adding my nutes. Also a lot of place now filter there waste water and in my area they have been finding chemicals from digested pharmaceuticals, it's very low but its there, mad loot to clean up. Check your areas water content before using it. I use distilled water until my seedlings turn into plants, because they have stored enough nutes until that stage nothing is needed. My well water is far to strong for them.