Been using citric acid for years. Didn't use any this year. Coincidently the flakes plugging the nozzles has stopped.We use the Hanna pocket pH meter. http://www.ebay.ca/itm/Hanna-Waterptroof-pH-Pen-HI-98127-pHep4-/380937512736?_trksid=p2054897.l4275