Nancy said:You could do a 20% water change and then add buffer (the kind that raises the pH) to the tank, a little at a time (as recommended).
Your pH registers very low - why don't you try your test on natural seawater and see what the results are. Your test kit may not be too accurate. And I find pH to be lower at night and higher as the day goes on. You should aim at 8.2, but some octo keepers recommend from 8.0 to 8.3.