Everything everyone else has already said is correct.
To gain the best advice you need to provide more information about your system. Temp, SG, Alk, Ca, Mg, method of Ca/Alk maintence, skimmer, circulation, bioload, etc.
IME, most hobby quality colormetric test kits have a have a range in error of about +/- 0.2 pH, so, if your worried about being accurate then buy a pH meter. In addition, the pH will raise and lower substantially during night and day cycles. So if your not testing at the same time each day your pH drop may be normal, depending on how big it is. It's best to use a calibrated pH meter and measure both at night and during the day. Even better, get a data logger, like an Aquacontroller and you can automatically record readings every hour or less and graph them.
I have a fuge and frag tank on an opposite light cycles as the main tank and automatically dose my 2-Part when the main tank lights are off. On average, my pH still drops from 8.2 to 8.0 or less at night. However, I'm also dealing with a much smaller system volume.
Hope some of that rant helps
