It there a possibility that your tank decor is raising your pH? Do you, perhaps, have natural rocks or coral or shells or something? That can raise pH.
One way to tell if it is something in the tank is to take straight tap water in a non-reactive container and just let it sit with some aeration for a day and test the pH. (The aeration it to drive off the dissolved CO2, which lowers the pH.) If its pH is lower than that of your tank, then something in the tank is raising pH.
My axolotls are in 8.2 pH, but this is higher than recommended. They seem happy, but they would probably be happier around 7.6-8.0.