R.o ph levels

hi , im in need of help with a dwc cloner , i have 2 air stones and a heater in a black 14lts bucket , heats is at 22c , any way im having issues with the ph always changing fast like + .7 over 15hrs or so , how can i better keep a stable ph for longer ?

R. O water is at 10ppm added some root nutes got it to 150ppm

Thanks guys