That's a pretty cool idea, and nice thing to do for people, especially anonymously. I was wondering how things were working out for you with regards to the
nutrients, but never got around to asking. I'm OK
nutrient wise right now, I don't expect to need any for a couple of months, now that I am down to 1 running system. It is getting hot again and don't want to start one that does not have a in ground (geothermal) reservoir, I only have one of these right now (my tomato's). So I have not been looking into more nutrients at this time.
I thought of doing the calculations using Miracle Grow to measure with. Not that it would be accurate, but it may be close in weight. But I don't have anything other than a analog bathroom scale to measure 1 pound of it with, and that would just leave too much room for error on both fronts to make it worthwhile. I would need to get 400 gallons (400 TBS) for $80 to break even. For me, there is shipping costs to consider also, there is nowhere to get hydroponic nutrients here in town.