Topping off a reservoir should be done when your nutrient reservoir drops lower then when it is first filled with water and nutrients. It should be done everyday if needed or every other day. Depending on how much water is lost from evaporation and plant transpiration. We always want to top off the nutrient reservoir with clean reverse osmosis water. NEVER use tap water for a reservoir top off. Tap water will change the NPK ratio’s and ultimately and negatively affect the nutrient solution. After clean reverse osmosis water is used to top off the nutrient reservoir. Now the nutrient solution needs the pH checked and adjusted if necessary. The reason we want to top off the nutrient reservoir, is because the more water that is lost to evaporation and transpiration means that the nutrient solution gets more concentrated. If the water is never topped off and continued to lose more water the plants might get nutrient burn from having too concentrated of a nutrient solution to handle for their roots. Rule of thumb; if the nutrient reservoir is a quarter less then full, top it off with clean water. Don’t go lower then a quarter of water lose in the nutrient reservoir. In certain stages of growth, too concentrated of a nutrient solution means you risk burning your plants. Always top off your nutrient reservoir to keep your plants happy.
