Hi DaveL, and thank you for the good catch. Yes, Promix is indeed a horse of a different color.
From their pdf:
High perlite content, peat-perlite based growing medium Canadian Sphagnum Peat Moss (65-75%)
pH Incubated:. . . . . . . . . . . . . . < 6.2
So yes, if he is growing in this then this is NOT soil. This is a specially designed grow medium intended to have a base pH of 6.2 This means that hydro rules apply, and any pH between 5.5-6.1 is valid, the center being 5.8, but the sweet spot being just a tad lower than that.
So, Hambone, first... are you a ham operator? I am, so I am curious. Second, forget everything I said about 6.3 pH being your sweetspot. Thanks to DaveL, we now have a very good explanation as to why your runoff ran so clear, and why there are folks advising you to pay attention to runoff. I went on and on about SOIL, not ProMix. Sorry for the confusion, but you are not in soil, and adding anything to the Promix to try to adjust it is ill advised. Follow DaveL's advice, or anyone else on here with a grow similar to yours, but my advice sadly has mostly to do with organic soil grows.