believe it or not the best water to use it RO water (reverse osmosis)
water has bacteria, viruses and hard metals in it, same with rain and tap water
distilled water only boils the water and collects the vapor, good for killing viruses and bacteria, but it doesn't remove metals
you don't have to go out and buy a ridiculously expensive RO system, just grab one on ebay, it will leak yes, its not top quality indeed, but it gets the water below 15ppm, i currently get 8-18 ppm
the feeding schedule on the back of your nutes (regardless of brand) is based on a ppm of nearly 0, but anything below 20 will suffice
the reason why a lot of plant problems occur is because the crap in tap/rain or distilled water is blocking nute uptake
...don't take my word for it, read a book
take a look
its in a book
Reading raaaaain booooow!