Quote Originally Posted by Mississippi Steve
Actually it was a question...having a little chemistry background, and playing around with my own grow, I would like to know what they are talking about.

BTW there are test strips that are available for aquarium water that will give you levels of a dozen chemicals and minerals for just a couple of bucks.
Wikipedia - T.D.S.

TDS is Total Dissolved Solids, or the measurement of ions and some other substances dissolved in the water. This is a measurement of the ions/salts in a soution. In your setup, this is basically measuring the amount of fertilizer/material in your water that your plant can/will absorb.

Quote from wiki: "The most common chemical constituents are calcium, phosphates, nitrates, sodium, potassium and chloride."

See the most common constituents? Notice the nitrates for N supply, phosphates for P, and potassium for K? Calcium and magnesium are also fairly common in this application, especially in 'harder' waters.