5.8 - 6.2 (oops missed the 5.8 by .1) No lower and lower is worse then higher and effects the plant faster.....Test and adjust every mourning at lights on!
FF or anyone else for that matter never "requires tap water"!!!
It requires Ca/Mg supplementation ! You DON'T know exactly what is in tap water....ANY water over 150ppm should NOT be used!
What a label says about tap or hard water CAN BE very "off",,,,Like I said, You (and they) DON'T know whats IN that water!!
I used my local well water at a seasonal swing of 146 - 152 ppm and did ok. After a year I was reviewing notes from my other facility and saw to big a difference in yields (hydro and soil). I installed a high pressure/speed RO system and everything evened out.
If you start from 0 ppm and add your Ca/Mg (CALi MAGic by GH) at 5ml per gallon. Then you add nutrition and pH.....There is less unknown "whatever" and the plant has only what it needs to grow! It grows better, bigger, faster,,,,,with only what it needs!
To much Ca? I would say no.....There are other compounds in the water giving it it's ppm.....That's what it looks like...and that could be related to the low pH!
Doc