under the normal parameters ideal is 7.4 - 7.6 tolerant is 6.5 - 8, now I class anything lower then 7 as bad because it means that the cycle can crash and that the axolotl can suffer anaemia, anything 7 - 7.4 not to bad, anything above 7.4 is good and means the tank water is better suited for a axolotl. when the ph was tested in 1978 - 1988 in the natural habitat it was found to be 7.4 - 8.2 so I wouldn't worry to much about it. I will add the literature