so, in looking at emerald harvest suggestions, which I don't agree with the flushing non sense, they don't even say anything about the water to use, from their site,
Flush the Plants – Once your plants have matured and are ready to harvest, you are not quite done yet. While all those nutrients that you used during the growth process are fantastic for the overall development of your plant, they can do real damage to the taste and smell. This is why it is imperative to “flush” the plant thoroughly before actually harvesting. All you need to do this is a reservoir full of pH balanced water and a plant flushing product.
and they recommend adding cal mag regardless of what water you use. Hmmm!