Well in tap water city tap I guess u r talking about not well water. Any who city tap water has chlorine and minerals and so on rain water isn't pure Hydrogen and Oxygen, it all depends where u live if u live in los angeles or Detroit then for get about it, the reason you have such pretty sunsets is because there is alot of pollution in the air that lights up when the sun looks orange/red and those molecules in the air gleam and reflect (Acid Rain) which is pretty much high concentration of sulfur in the air there for water.
Save ur time and plants and go into a Walmart dodging the Nascar billys and buy yourself some 37cents a gallon Reverse Osmosis water.