Clayboy I am on my first hydro grow too and experienced the same thing. After a while ( a week or so) it should stabilize within a couple of points or so of your target...mine climbs to 6.1 or so (my target is 5.8) but for a while it was hitting 6.7 every night.

Here is what Weedhound said when I asked "why?" too:

"I was told that the process of destruction of the plant tissue itself causes ph to rise. For me, it's usually the first sign that I have done something wrong. Not only does it go up quickly, its hard to bring back down and just doesn't want to stay there until the plant stabilizes. This is not like the rising ph at the end of flowering which is normal and due to high nutrient needs."

Make sure your pH pen is calibrated and learn to trust it - you'll be fine.