I found that opening my windows raises my pH 2-3 tenths. My pH is stable (windows closed) at 7.8-7.9 There is something to be said about the overall air quality in one's home. Ambient CO2 levels seem to have a direct effect on pH. The problem is that there aren't many days in which I want my windows open.
I too have been told (and have read by the experts) that my overall levels aren't all that bad. But they also say that it is important to check the calibration of your probe before trying anything to correct it....Personally, I never exclude the possibility that there could be an issue with the age and reliability of the calibration liquid. Most NIST lab grade pH solutions that I've worked with expire at 2 years. I don't think any store bought fluids will be any better than that.
Also...is it possible that I have diluted my solution along the way? It is necessary to clean the probe before calibration. It is important to rinse the probe with the same solution you intend to dip the probe in. If I just rinsed my probe (after cleaning it) with some DI water, it has to be rinsed again with my pH solution. If I don't do this, I will gradually dilute my calibration solution with the excess water on the probe.
Probes need to be cleaned and calibration checked every 30 days. This is an industry standard and will not be reliable unless this is done.
Probes do not have a unlimited lifespan. Many probes need to be replaced after only a year of service. Faulty probes that are near the end of their useable limit will begin to give erratic readings.
Some aquarium probe manufactures supply the wrong pH solutions. I just talked to a friend who got a leading aquarium pH probe manufactures' instrument and they only supplied 4 and 7 pH fluid. She didn't know any different so she calibrated it and just plunked it in the aqaurium. This will most definitely give false readings when dropped into a saltwater fish tank. For saltwater tanks, the probe must be calibrated between 7 and 10 or 7 and 9...whichever your probe manufacture suggests. The calibration slope for 'most' pH calibrators is different from 4 to 7 than it is for 7 to 10. If you calibrate your probe in either of these ranges, it will not be accurate in the other range. It will most likely produce an error of 2 to 3 tenths. Some pH meters compensate for this but they are more than likely industrial or lab grade and much more expensive. We monitor at 8, so we must calibrate below at 7 and above at 9 or 10.
I have some first hand experience with this and from what I've seen so far, calibration solutions are not 100% reliable. Many people do not understand the errors that could result if the proper techniques aren't utilized. These instruments are consumer grade instruments and have no need to adhere to industry standards. The margin for error is increased because of these facts.
But I'd just like to say that in light of all this, dosing your tank should be the absolute last resort. The problem could be right at the tip of your probe.