What is the best type of water to start with?
Most growers will have no problems using tap water. You can have the tap water analyzed to determine if it will give you problems or you can buy a good filter and start with a totally clean slate.
The most common problems with tap water will be:
pH -- usually caused by lime or calcium in the groundwater supply
high parts per million (lots of dissolved junk in it) -- yum, city water. Certain elements dissolved in the tap water may cause lockups and then deficiencies if you are using chemical fertilizers.
chloramine -- used in wastewater treatment
The best filter to use is called Reverse-Osmosis. Under-the-sink models can be purchased at any home improvement store for around US$200. They will require cleaning and filter replacement around once a year, depending on how many gallons you put through it. The typical model can clean 50 gallons of tap water down to 0 ppm and 7 pH daily.
** The downside to R/O is the amount of waste water it produces in the process. This loss to drain can be cut by up to 85% with the addition of a 'permeate pump' to the R/O system, it is an easy installation that should require no tools in most cases. Permeate pumps require no electricity and can be purchased online or at your local water filter supply company.
Permeate pumps are the newest breakthrough in Reverse Osmosis System Technology. Designed to operate as a non-electrical energy recovery device, the Permeate Pump uses the available energy from the brine (waste water out of the RO Unit) water after the flow restrictor to essentially force purified water into the RO Storage Tank. The permeate Pump eliminates the need for hydraulic shut-off valves and can be used for replacing booster pumps in well water applications. The Permeate Pump advanced technology is capable of bringing the tank up to incoming water pressure and shutting the RO System down when the Storage Tank reaches full capacity.
Distilled water is also good to use. Mountain or spring water will not be 0 ppm as it will have dissolved minerals. Using cheap home filters like Brita pitchers is pointless as they do little to clean the water.
Note: it's not advisable to use plain R/O water in a hydroponic medium for flushing. The shock of 0 ppm will start leaching nutrients from the roots. Clearing should be done around 100 ppm instead.