How often do you change your reservoir? I only change mine every couple of weeks, because my system is really stable. I don’t see much fluctuation in pH, and my res is filled with beneficial bacteria that I keep happy with a biofilter. For me, it’s a matter of effort vs. reward and my garden generally rewards my minimal efforts! But over on the GHE blog, the question was addressed by their head of R&D, William Textier-

Question: “How often should you change the nutrient solution?”

Answer: “It depends on the quality/price ratio between nutrient costs and crop yield and return. If this is an important ratio for you, it is best to replace your nutritive solution every 10 days. If it is not, you can keep it up to 3 weeks without problem, providing you regularly verify your pH and EC levels, and adjust your solution accordingly. Do not discard your old solution. If you have home potted plants, or a garden, use the old solution, with added tap water, on your soil plants.”