Your 7.7 reading could have just been off or you might have had too much water in the test which would give you a lower reading.
Your ph will read highest at night just before "lights out" and lowest in the morning before turning on your lights.
If your tank has glass tops that will trap CO2
which will lower your ph. Short lighting schedules (3 hours or less) can lower your ph over time.
usually will keep your ph in line but you may have to add a little ph buffer to your pwc
water depending on the salt brand you use. My salt always tests a little low so I use 1/2 a teaspoon of Seachem Marine Buffer
with each pwc
All tests should be compared to the color chart against a white background. 15 ppm
is 100% better then where you were and will come down more eventually. Striving for 0 ppm
is nearly impossible but at least keep it 10 ppm
or less if possible.
If doing pwc
larger then 40% then you do run the risk of hurting the livestock. Matching your change water's ph/temp/sg
perfectly to the main is critical either way.