Distilled Water... why use it? Plants can only take a certain ammount of solids/ salts in their feeding water before they are overwhelmed. Different plants/ strains are different, but let's just say our plant likes 1200 ppm worth of 'stuff' dissolved in the water. Tap water starts off with a ppm of 100-300 I beleive depending where you live... it contains all sorts of stuff (chlorine, calcium, etc...). So you can add 1100-900 ppm worth of nutrients to bring the total up to 1200. If you start with Distilled Water it has a ppm around 0, so that you can add your nutrients all the way to 1200 ppm. Plus it is just cleaner than tap. I ended up getting an Reverse Osmosis filter and use RO water.
Also Tap water has to be left out to sit for 24 hr at least to let the chlorine evaporate... and I have cats... so they would be swimming in it if I had tanks of tap water sitting around.