Why does my pH in my tank fluctuate and how can this be fixed?

I added rocks I got from a quarry which had limestone or calcium in them. I have taken the rocks out once I did a test on the rocks to realize they were doing this. Now with no rocks in it the pH still changes from very acidic to alkaline every time. Is it due to deposits in the tank or filter? I've done a complete water change and even rinsed off the gravel to start from scratch. The tap water is extremely acidic so I wanted to see if the tank would still alter the pH to alkaline and it did it again. Doing the testing went from yellow to blue without any alteration to what was in the tank. Please help!