pH in DWC too high, how can I low it naturally?

I find bottled is more stable then using natural methods. If you do go natural try to get a mix as it will be more stable. Most PH down will contain phosphoric acid, citric acid, and mono ammonium phosphate If your getting a big swing then you might have some light leaks creating growth in your water. In dwc you should have your PH range between 5.5-6.5 and mix your solution within the range of 5.8-6.2. Generally your feed and ph will go in opposite directions. So if your over feeding then your EC/PPM will go up and your PH will go down. Under feeding EC will go down and PH will go up. But with a plant your size It would not change much. Food looks a little strong but I am not familiar with that brand. Generally you want to use 1/4-1/2 of what it says on the bottle/feed chart. As your plant gets bigger and your adding more food your PH will stabilize because you have more minerals in the water to keep it stable.