when reading ph you use the the standard ph test, the test range is 6 - 7.6 if the test indicates 7.6 then a high ph test is done which has a test range of 7.4-8.8, the two tests combined give a test range of 6-8.8, because your result for the first test was 7 there is no need to perform a high test.
as to your ph ideally the ph should be 7.4-7.6 and at ph 7 and lower the biological filtration can start to struggle to remove/convert ammonia and nitrites, a low ph can also be a sign of low kh, use bicarbonate of soda to slowly increase the ph to 7.4-7.6, it might be a good idea to test kh gh as the water may require more salts/minerals to keep your axolotl healthy and happy.
info on levels..
Water Quality Explained: How It Can Affect Your Axolotl's Health - WSAVA 2015 Congress - VIN
info on water conditions..
Axolotls - Requirements & Water Conditions in Captivity