Quote Originally Posted by the image reaper
your pH is rising primarily from the buffering effects of the rockwool and hydroton ... you have to pre-soak rockwool for a few days before using it, to allow it's pH to settle ... hydroton also contributes alkalinity ... I try to take advantage of the pH rise, by being able to run a little 'hotter' nute feeding :jointsmile:
IR hit it: your grow media is most likely causing the fluctuation. Throw your media into a bubble bucket with some pH'ed (5.6-5.8) nutrient solution and let it bubble out. Use a weak nutrient solution (like 10-25% of full strength) and check the pH every few hours. When you go a day without the pH rising (or falling) much, you're done.