It is probably a winter oxygenation issue. To be sure, you can take a glass of your water, aerate it for an hour (outside is best) and then retest, and I bet the PH rises significantly. You can improve surface agitation, or skimming with an air line run outdoors. You could also drip kalk to try to raise ph, but if you don't need the alk/calc it isn't really a good solution. If everything is healthy I wouldn't worry about it too much.