Thanks for your input David. It's a fresh tank with town water degassed for a few days to remove the chlorine.We use a product called z-7 it's a probiotic enzyme that keeps our lines and tanks clean and free of biofilm. We've been using it for months now with great results ie. much whiter roots /reduced build up in our microlines/healthier plants and better yields. The only difference here I can think of is the temp has dropped from high 70s to high 60s F which has cooled the nutrient water to the low 60's.