Promix is "soilless", which I gather means it's not amended with all the various organic and mineral components that true soil has; true soil is a pretty diverse mix of things... I'm pretty sure BioBizz light is very similar, and I'd treat the pH range the same, unless BioBizz says otherwise (I don;'t use soilless)..... As you see, our mate
@Ribbzzy is a BioBizz sensei
- just look at all the
phat chicks he hangs out with!
..... if you're using AN, don't mess with pH adjustment at all according to their instructions,... BTW, what's the water source you use, pH/ppm's? See if you use very hard water, (AN rec's not to at all), it can overwhelm the buffering capacity AN has in solution,.. and keep in mind the "self-buffering" deal applies only to the solution, not when it gets into the pots... The medium's do have buffering minerals in them, right? So unless what you put in is pretty far off the proper range, don't force it down/up if it's close, say +/- 0.2 give or take... there's more availability flexibility in soilless vs. hydro, the chemistry in aqueous solution is worlds different from what even soilless behaves like,....
Ca-Mg should have some carbonates in it, and that's what's directly involved in pH buffering, not the free Ca++ itself,... CO3-- will instantly snag up a free H+, turning into bicarbonate HCO3-, which is how it reduces acidity.... Anyway that's why Ca-Mg goes in first, to get the buffering chemistry settled first before you potentially add any adjusters, otherwise the two at more or less the same time will not have time to react and pH settle and you may get wonky readings!
Odd, that your AN solution is coming out that low,.. I suspect you're using RO/Di water? OR you pH meter is off calibration,... you need to be anal about that, storing it in the right solution, rinsing clean after each use, and calibrating often, at least weekly.... RO/Di water will also swing wildly in pH with even small inputs because there's no buffering at all in there, no minerals....