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It's a constant battle with ph on my tank keeping above 7.9 I dose alk and as per the apex you can watch the ph spike 8.4 then gradually back to 7.8-7.9 within a few days ??? My test results are a mirror image of yours except my mg is about 1600ppm
I wouldn't chase the pH, it's going to be a little low any time of year the house is closed up due to weather, causing higher levels of CO2 in the house hold atmosphere. Keep your Ca and alk in acceptable ranges, and you'll be fine. You could however, run an air line from your skimmer to an outside air source(if practicable), or add a CO2 scrubber (soda lime) to it. Although it's not suggested that you use it solely for this purpose, using kalk in your system will help keep your Ca and alk up, and in doing so will utilize some of the CO2 in your system, helping to mitigate this issue.
I run both a CO2 scrubber, and drip kalk in my system overnight, and my pH will still get down to 7.9. I use a properly calibrated lab grade probe on a pinpoint monitor, so I think its about as accurate as you can get with hobby grade equipment.