As I understand it, blackwater extract is largely tannins, not fertilizers. It will really throw off the CO2 measurement if you try to just measure the water pH and KH, since it is acidic. I agree with Flora, read up on the estimative index method of fertilizing - it is easy to do, easy to understand, and foolproof. And, I hope you can post a photo of the blown glass drop checker. That should be very interesting.