How often should I change my reservoir water? 

We recommend changing out your reservoir water completely every 7 – 10 days. Even though you can top off evaporated water by measuring and not exceeding overall nutrient concentrations, you can’t measure the ratio of different nutrients left in the water after plants have used part of them up. (For instance, plants may need / deplete more nitrogen during some growing stages.) 

One of the things you never find in other manuals is the following: We have told you before that the absorption of nutrients is an active process and that plants can, within certain limits, decide what and how much they absorb with their roots. Suppose that a nutrient solution contains nutrient A and nutrient B in equal amounts. The plant grows fine but it happens to use some more of A than of B. After some time the EC has dropped and is adjusted with new nutrients that again contain equal amounts of A and B. But more has been used of A, so after adjusting, the solution contains more of B than of A. If this continues for a while the solution will contain too much of B and too little of A whereas the EC has the right value. With the EC you determine the concentration of nutrients but not WHICH nutrients. Besides, the plant also expels certain toxins through the roots into the drainage water. For these reasons we strongly advise you not to use the drainage water a second time.