Just some thoughts.. RO is fine, but ya gotta be careful with that too, true RO water is acidic. Alot of these formulas if you blend with RO, and they are somewhat calibrated for tap , you end up with PH lower than you want.
This statement is very wrong in relation to our pH needs.....Water that has been produced by a properly functioning reverse osmosis system is some of the purest water available, yet often RO users are surprised and a little alarmed to find that their very clean water is also “acidic”, that is, that it has a low pH. Neutral pH is defined as 7.00, but RO water typically shows a pH of between 5.00 and 6.00. (Mine comes out at 6.0 and all I do is add my nutrients and water my soil. I NEVER have pH issues!) This amount when nutrients are added tend to balance out to our needed levels. Of course you should always pH your solutions to your exact needs.
6.5 for soil grows and between 5.9 and 6.2 for hydro and soiless (5.9 or 6.0 being your target).
Why then does RO water test to have a low pH? The very purity that means it is neutral also makes it very sensitive to the addition of other substances. Technically speaking, RO water has little or no buffering capacity. That means that the addition of even a small amount of acid will have a large effect on pH.
Reverse osmosis membranes do not remove gasses, such as carbon dioxide in water. Also, when RO water is exposed to the air, a small amount of carbon dioxide will begin to dissolve in the water. So RO product water has the buffering alkalinity chemicals removed and the acid causing gasses remaining.
Having low pH RO water should not be of concern because the “acidity” in RO water is very weak. In order to bring RO water to a neutral pH, it takes only a tiny amount of base. For example, if you had a glass of RO water with a pH of 5.0 and you added 2-3 specks of baking soda (a base) that would likely neutralize the tiny amount of acidity in the water.
So then, do you see what I mean?
Get an RO and add the Ca/Mg......Tap water has FAR more disadvantages over RO. You simply DO NOT know exactly WHAT is in it! Remember what Snaps said?
"Get me some clean h2o and I know exactly what I'm working with to start as far as calcium, iron and bacteria."
Just as a point of knowledge. I have 146 to 149 seasonally dependent ppm in my well water. Sounds good right?
In all media, I get HIGHER yields using RO then the "usable" well water (That is true for hydro and soil grows). The difference is more then enough for me to have paid good money for the RO. Hell it was paid for in the higher yield of ONE grow!
Tip of the hat to PURE TAP for the RO facts I supplied here.