In related news..
I used RO and my usual 1part and saw PH around 4.0 which the coco plants liked, once I stopped correcting it. The run-off was about 7 iirc.
I was simply seeing the PH of the components, as the RO has no buffering action. The food isn't even a thousandth of the feed though. What's in your substrate is a much bigger part of the recipe. Some of which gets used up, which your replacing. So while it may seem your slowing acidifying the medium, you may actually be stopping it getting more alkaline.
I'm just using a little reasoning, not solid science. But many people use that food believing that to correct PH for soil is wrong. So that's a lot of testing right there.