After battling with pH for a long time, and doing all of the recommended things (airline outside, skimmer, refugium, c02 scrubber, house plants, higher water movement, etc) what finally worked was dosing kalk at night. I dose about what I evaporate in a day, but through a doser. My pH was bottoming out at 7.8 and in the last 24 hours it bottomed at 8.1 with an 8.2 average.
I am new to doing this on my display, but so far, this is the only thing in the last three years that has helped.
I know that running an airline outside really fixes the issue for most people, but I think, and this is my opinion, that some of our environments just don't have the best outside air. I live pretty close to a very large freeway and a mile away from a power station. The outside air just wasn't helping.