How to fix pH for the long term?

That kind of daily crash from ~6.2 to 4.5 in DWC isn’t something I’d keep correcting with SilicX. The drop itself is the clue. A healthy reservoir can drift, but that’s a huge swing. I’d first calibrate the meter, then track pH + EC + water level together for 24h. If EC is rising while water level falls and pH crashes, the solution may simply be too strong for what the plant is taking up. Also check reservoir temperature/aeration and roots for discoloration, slime or smell, because root-zone microbial activity can cause severe pH instability. With a 15 L reservoir I’d probably do a fresh change rather than keep adding products every day, start somewhat lighter than the full chart, and see whether it stabilizes around the normal hydro range. Fix the reason for the drift rather than repeatedly fixing the number.