Hi Mark and welcome to SD. There are many factors that could cause your PH to rise. The first that comes to mind is the fact that the water in the tank is aerated and therefore the PH will rise due to the CO2 gasses being bubbled off. That is why many of us (myself included) age our water because the ph out of tap is lower than the ph once aged. I have a feeling this is the rise you are seeing. You may need to start aging your water to bubble off the c02
Also, there are various types of rock that could cause a rise in PH because they leech mineral hardness into the water. what do you have in your tank other than water and fish?