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That last part is interesting to me, how would you test the nutrient solution after circulating? Would you just pay a lab to run the analysis on what's left or evaporate the solution to increase concentration and then try to precipitate the ions left in the solution and weight those?

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You'd pay a lab about $45, same as I did for my quality control check. A few of the ions--like nitrate or ammonium--will probably get tested with some kind of color-change reaction, and quantified with a visible-light spectrophotometer. The rest will be from ICP (inductively coupled plasma), a precisely-quantified version of the same effect as where different elements make a flame turn different colors.

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