Very true & Good advice.

I would just like to add that it's beneficial to take an EC reading of your tap water and ADD that value to your target EC. Example: If tap water EC=0.4 and your target=1.2, then your new target will be 1.6.
That way you actually add the tap water minerals (Ca and Mg) to your hydro nutrients.

Yes, I would normally say what greystoke said, but aren't you saying that you proved that to be untrue? You had to add nearly the same amount of nutrient in both cases? I've also always added the original EC if I use tap. I rarely use tap now since I use rainwater, but now I'm interested in seeing how much difference there is and how inaccurate simply adding EC of tap to what the EC would be in rainwater with nutrients. Did you try this more than once to make sure there was no anomaly throwing off your results?

If you ignore the tap water EC, then the calcium/magnesium in the tap water are going to replace some of the salts in the nutrient mix, while adding salt mix to reach a certain EC, because you're starting off at a handicap. In other words, you get a lower nutrient mix. Lower in nitrate, phosphate and potassium. I don't think that's a good idea

Re: Joe.jr317's comment

I make up a huge batch of nutrient when I make it (like about 30 gallons!). Here's what I did:

First, I used tap water and added nutrient concentrate (home-made from compost tea using anerobically-digested chicken manure, which is brewed using tap water also). I filled the big plastic tub I use to store the stuff in with tap water and added nutrient concentrate until the EC reading was between 1.2 and 1.4 measuring things with a Bluelab Truncheon and assuring that the 1.2 light blinked more than the 1.4, indicating the reading is closer to 1.2 than 1.4.

I used up all but about a gallon of this batch and made up another only this batch used R/O water for the concentrate brew as well as the total mixture brew. I drew off 2 gallons of R/O every day from my home R/O unit to enable my having enough water to do it. I stored the water in a plastic trash can with a poly-bag liner and the lid on.

I noticed that when the EC reading was finally at 1.2, having started at zero conductivity, I had added the same amount per gallon of nutrient concentrate (1200 ml / gallon) as when I was using the tap water.

My storage bin has a fairly-large fish tank pump and a 1-foot-long air stone, which sits at the bottom and blows bubbles through the stuff 24/7. Ii do this to give my plants grown in static containers filled with perlite a little oxygen "boost" when I replenish their reservoir puddles as they transpire the water inside their 10" x 20" plastic trays sitting on the concrete floor of my greenhouse.

I've only done one batch each way, so maybe before you go whole-hog on this, either wait until I do more research or do your own using smaller water volumes. I dunno.

Actually, I intend on using tap water from here on in, so I can get the added benefit of the dissolved calcium and magnesium, which the R/O trashes.

I was as amazed as you are that each type water took approximately the same amount of nutrient concentrate. Maybe it's magic!

I think I will try this on a 2 gallon level. If I get time, I'll do it today. the EC of our tap water is usually around the .8 mark. When I used tap before, I just assumed I should be subtracting the .8 to determine nutrient levels.