ph and Alkalinity

When I first went to look at the 400 Gallon tank that I bought, I went over it and noticed that the calcium reactor was running and that the effluent was plugged into a quick connect port on the skimmer. The catch was that there was no C02 plugged in. The ph was rock solid at 8.3 on the ph monitor. It took me 2 weeks to get the tank home and setup. I set it up exactly as it was and watched the ph rise from 7.5 to 8.2 over the course of a week.  I was worried that I was getting a false reading and so I calibrated the ph probe and it's right. I am not dosing. How is it that my ph is on the rise?