Check the dates on the bottles, as one of them might have been sitting at the LFS
for a while. I trust the AP tests, and you can check the date of manufacture by looking at the last digits of the lot number. AP says they are good for about a year after the manuf. date. Unless you are breeding a tricky fish, I would get one test and stick with it, because even if the exact number is off, you can spot a change, which is really the danger associated with pH. As long as the number remains the same you can assume your pH is stable. I am getting an electronic pH monitor (http://www.aquabid.com/cgi-bin/aucti...its&1068404406
) and I am going to compare the results of this monitor with the AP test kit.