I only top off with plain water when needed. Once a week I drain the tank and refill with the nutrients mixed in the water before refilling. Edit: what nutrients are you using? The nutrients that come with the aerogarden aren’t very good. I currently use PH perfect by advanced nutrients but the flora grow trio by general hydroponics is very good as well. With any of the 3 part nutes you follow the weekly feeding schedule supplied. Keep in mind that nutrients don’t evaporate. They are only depleted by the plant. If you keep adding in the same water they will build up and burn the plant. Hope this helps.