Will you at least look at my question? its about my fish tanks ph?

Okay the ph of my well water is 6.2 and the ph of my aquarium tank is above 7.6. It has blue gravel and two ornaments I haven't put anything to change the ph. But my sister put one of those paper thermometers inside the tank and I was wondering if that was the cause and I changed 50 percent of the water and it helped but it went back up even after the thermometer had been out of the tank I just want to know why this is happening and how to fix it