If your tap is 7.0 and it goes to 8.0 that is a serious jump. Plants and fish will never cause a ph change like that. At least a 1.0 difference. If your not already injecting co2 then plants are not using the abundant supply to change parameters like that. If your tap is 7.0, it is 7.0. To change that, your tank has to be introduced to more acidic conditions or more alkaline conditions in order to swing one way or the other. I am sure its something in your hardscaping. Soil/substrate, driftwood (usually acidic though) or rocks.
24hrs is not enough for the testing of your stones. Did you do a vinegar or muratic acid test on your rocks before putting them in your tank. I would isolate and test your rocks for at least a month. I am currently testing 5 different rocks to get a better idea what parameter changes will happen. After 24hrs of testing mine didn't not shift either. But after 3 weeks, there are some changes happening.