Assuming your a soil grower, a PH of 6.8-7.1 would be sufficient, and cold tap water works just fine.
Flush your plant with enough water to equal 3 times the amount of soil. So if you've got them in 5 gallon buckets, 15 gallons of water will do the trick. 1 gallon = 3 gallon, etc....
After flushing, you'll notice(as pmoore pointed out) the fan leaves will begin yellowing, and dying. This is because you've flushed all the nutrients out of the soil and roots, and now the plant is in search of food. The plant stores most of its food in its fan leaves, so it turns to them for nutes, sucking the life out of em' so to speak.
A good rule of thumb is to wait 2 weeks after flushing before harvesting, but in dire times, 8 days after flush will work just fine.