Help with mystery PH spikes

[BC198722]

I’m having issues with my PH and I really need help. In December I set up a new shrimp tank. It’s an 11 gallon tank. The hardscape is dragon stone and I have a few pieces of nolos wood and one piece of drift wood. The substrate is fluval shrimp and pant. The tank is heavily planted, mostly with Cuba carpet and some vallisneria spiralis. I do not add CO2 but I do add a pump of shrimp safe fertilizer twice a week (Brightwell Aquatics shrimp Florian multi).I do a weekly 10% water change and top off the tank when needed (usually twice a week). The shrimp are fed blanched vegetables, wafers and sometimes bold worms 2-3 times a week. The PH from my tap water is 7.4 and for awhile the tank sat between 7.2 to 7.4. Now it’s consistently 8.0 to 8.2. I’ve tried everything to control the PH and I’m failing miserably. I’ve added more wood and Indian almond leaves but they don’t have an impact. I reduced the aquarium light time and took out my extra air stone but to no avail. The only thing that does help are larger water changes but it doesn’t last long and it agitates the shrimp. I’ve tried smaller daily water changes but that doesn’t help either. I’m hesitant to use PH down because I don’t want the PH to dip too suddenly and I really want to tackle the underlying root cause. My final thought is it might be the dragon stone but I’ve never had issues with dragon stone before and I hear it’s PH neutral. Any advice would be welcome.