If we are talking hydroponics I've always found that staying between 5.6-6.1 does me peachy. Soil and soil-less mixes are a different story.

You are stating ebb and flow in rockwool......that's hydroponics You most likely SHOULD be using CalMag but I'm not positive there because I'm not really familiar with your nutes and some seem to have calcium and magnesium in them and some dont. If yours don't, you'll need to add it. Yes you can use your tap water and if the ph of it remains stable (put some in a cup, adjust with ph up and down, let it sit overnite and see how stable the ph remains. We use RO water (with CalMag) because of ph issues with our own water.

I personally would stick with the distilled and find out if you should be adding CalMag. I am assuming you are using true ph up and down for plants and not something made for aquariums or lemon juice or whatever. That will make a difference over time as well but that's just an aside.
Weedhound Reviewed by Weedhound on . ph, the "correct" range I've heard alot of stuff here and there, but I can never get a definite answer for this question. What should the ph be? I've heard 5.1 - 5.8, 5.2 being optimal. Local hydro shop owner says at those ph's, rockwool actually "melts??" Is this true? He's telling me to keep it 5.8 - 6.8, aiming for 6 and 6.3 being optimal. This is my problem. Rating: 5