Some growers report purified or distilled water helps their plants grow faster. Perhaps due to sodium and heavy metals found in hard water that are not present in purified water. Hard water tends to build up alkaline salt deposits in soil that lockup trace minerals, and cause iron, copper and zinc deficiencies. There are several types of purified water, but many are not free of minerals that could be causing salt buildup over an extended period of time.
Tap water comes in two flavors. Hot and cold. The cold pipe has less calcium and sodium buildup in it, and should be freer of sediment once the water has been turned on and allowed to flow for 30 seconds. Hot water will have rust, lead deposits, and lots of sodium and calcium, so much so, you will see it easily. Use only the amount of hot water needed to make the water the correct temperature (70-80 F). Tap water filtered through a carbon (charcoal) filter will be free of chlorine and most large particles, but will still contain dissolved solids such as sodium and heavy metals (lead, arsenic, nickel, etc.).
Purified bottled water will be either Reverse Osmosis or some form of carbon/sediment filtered water. When purchasing water at a store, unless it says RO or Distilled, don not bother buying it. It could still have the same dissolved solids and heavy metals your tap water has.
Article written 2004 and featured on Grow-Marijuana-Seeds.com