My tank started to experience very low PH down to 7.5 PH one morning even with Kalk dosing, normal range was 7.6-7.9. After some simple aeration tests showed that the water would go to 8.2 PH. I bought two large air stones from the hydroponics store and placed them in a covered area of my sump. PH has never been below 8.0 at night now with Kalk dosing to a high of 8.2, and without Kalk bottoms out at 7.9 PH (if my ATO water is just plain RO.)
I would definitely take a 500ml sample and aerate it with a small airstone outside for 1 hour and remeasure its PH.. If that drops then do the same thing with a new sample indoors, aerate for 1 hour and check the PH. In my tests it easily showed that my problem was not so much my indoor air but a lack of surface agitation to get the proper gas exchange.
I agree with others, I didn't show many real problems with Low PH from gas exchange. My tank slowed up greatly once when I didn't do a WC for a while (few months,) after just a small 5% change everything radically started growing again... I am not sure what your WC schedule is like, and its typically the horse that is beat dead, but always worth the mention
