How measure PH?

[Brent F]

I have been using the Elos kit to test PH and the test comes out red which is closest to the light purple on their colour chart - PH 8.3

Since the red is quite different than the light purple I thought I'd try a different brand of kit and make sure I am interpreting the test correctly. I bought a Nutrafin kit and it says my PH is 7.5

I am amazed at the huge range from one brand to another.

My alkalinity has been dropping steadily so I was looking at buffering it. Some products lower PH while others raise it. Hard to tell what to use when I don’t know what the PH is.

How do you guys measure PH accurately? Should I get a probe?