pH creeping up to 8.1
I have my tank up and running for about 2 months now. Everything is going smoothly; it's cycled, plants growing, shrimp and snails seem happy. I'd really like to add some fish soon, but the final issue I'm having trouble sorting out is my pH. My tap water is usually around 7.4, which I mix with RO water from a lab bringing the pH down to around 7.0 before adding to the tank. However, even just 2 days after a 30% WC, the pH is up around 8 or so. I'm not too sure what the issue could be, but I have two theories. I didn't measure pH continuously after I set up the tank, I think I measured for about 2-3 weeks and it seemed stable so I stopped.
1: It seems like my pH increase coincided with me adding the filter. I wonder if the "homemade" fine filter pad I made from foam I got at the hardware store could be leeching something into the water, causing an increased pH. I measured pH 5 days after setting up the filter and it was around 7.2 (so normal) but then 14 days after setting up the filter it was up to 7.8. I've ordered official filter pads from eheim to address this.
2: Something else in the scape? the soil or the rocks? I have only lava rocks and those are buried quite deep beneath my substrate for structural purposes so..
My water params:
Ammonia: 0 ppm
Nitrite: 0 ppm
Nitrate: 0 ppm
Phosphate: 10 ppm
Any help or comments appreciated. Thanks!