I am using a passive system with air stones. I am growing lettuce and basil. Whenever I change my nut solution my roots turn brown. I remove the dying roots and in a short period of time new nice white growth appears. I am using Masterblend with micro nutrients. I tried an organic product from my local hydro store and didn't like the results so I went back to Masterblend. And I have cleaned out my reservoir disinfected and started new and when I change nuts the roots still turn brown and begin dying.