First off, I'd very strongly suggest doing an ammonia test.
For a KH of 10, a pH of 7.2 seems unusually low.
Also, 0 nitrates sounds a little odd.
How did you cycle the tank?
Do you have plants? If so, how many, and do you have more than standard lighting?
Add any chemicals to your water other than dechlorinator (particularly "pH reducing" chemicals)?