THIS. Does your system fill up by itself? If not, run your RO to a float valve in your Rez so it always maintains the same water to nute ratio and your ppm should stabilize. If not you probably have something bad growing in your water.When your plants drink more water the nutes becomes concentrated so ph drops.
When your plants drink more nutes there is more water so ph will raise. You want your plants to drink equal parts of nutes and water.
What's in your tap water ? Chloramine? did you let your water sit around to evaporate Chlorine?Ph can drift when chems evporate.
I wouldn't use silica blast as ph up.
How is your plant doing? is she healthy ?