I have a ph that keeps returning back to 6.5, why ? The plants are under 18hr 600w Sodium lights. In pots with clay pebbles fed through dripper tubes every 3 hrs. Using Canna Aqua Vega. Have flushed system with fresh water. I try and keep the ph at 5.8, but it returns back to 6.5 after a few hrs. Have recaliabrated PH meter, buts still get the same results. The plants are now showing signs of yellowing leaves.