Nutrient depletion and replenishment
I had thought that I might be able to replenish the nutrients as they are used by the plants, but I suspect the ratios of the various components are constantly changing. So, to restore the nutrient mix to its original levels, a chemical analysis would be required. Perhaps in a large operation, replenishing the nutrient solution may be cost effective. My system has around 100 gallons nutrient mix that circulates through it. I use the special hydroponic mix sold by Southern Agriculture. It comes in 25 pound bags and runs under $30 dollars. For mixing up 50 gallons of mutrients, I use 1 cup of this product along with 1/4 cup of epsom salt, and 1/2 cup of calcium nitrate. I get a TDS reading of around 800, up from a reading of 130 for city tap water. I haven't figured out the exact cost per gallon, but I think it comes out to be less than 5 cents a gallon.
The cost is low enough that I think the best procedure will be to dump the entire mix once a week and mix up a new batch. I found small bags (4 pounds) of calcium nitrate on ebay, and epsom salts can be found in any big store.