If you get Lake Michigan water (I see you're from Chicago) your alkalinity should be 100 ml/L, that is a good buffering capacity.
A drop in pH is usually from decaying organics.
You may be feeding too much, or not doing enough water changes and cleaning up waste from the substrate.
Small frequent water changes would help stability, if it were me, I'd do a partial water change every other, or even better, every day, where I'd siphon the substrate with each change.
You don't mention filtration, but I would be cleaning out the mechanical filtration media at the same rate.
A drop in pH is usually from decaying organics.
You may be feeding too much, or not doing enough water changes and cleaning up waste from the substrate.
Small frequent water changes would help stability, if it were me, I'd do a partial water change every other, or even better, every day, where I'd siphon the substrate with each change.
You don't mention filtration, but I would be cleaning out the mechanical filtration media at the same rate.
