Originally Posted by TwinDad
I should have made it a bit clear I was trying to see if I understood what the others said. It's good to know what ratio got you 7.4. I'm contemplating doing what your doing to lower my ph. So I wanted to get my head around what was suggested.
My ph is stable at 7.8. Really hard to move even though my GH is 44-77ppm. According to my water company. I'd like to get to 7.2. I might experiments like you to get it lower, my only concern is it going up and down. Mine isn't going lower over time, it is staying the same. I left tap in a bucket for a week. 7.8 the same as when poured.
Things will be different, sometimes significantly so, from aquarium to aquarium. It is not the tap water and distilled water mix on its own, but the effect of the biology of the aquarium. Letting a pail of mixed water sit for days will likely not show any change in pH because there is nothing acting upon the water chemistry [though in some situations there could be, such as dissolved CO2, which would normally gas out within 24 hours]. But once in an aquarium with fish, plants, bacteria--it may slowly or suddenly shift, and more likely down.
So, once the water is mixed and showing the desired pH, do a water change (around 30%) and monitor the pH in the aquarium over several days. And so on, depending upon the results. But be aware that as the acid increases or the KH decreases, at some point the buffering capacity may be exhausted and the pH can then very suddenly and sharply drop, what we term a pH crash. This can kill fish and plants.
Water in an aquarium will naturally acidify over time due to the biology. The pH will lower accordingly. The extent to which this occurs depends upon other factors, such as the KH of the water, items in the aquarium that may affect water chemistry (wood, rock, calcareous gravels/sand, leaves, peat, carbon, etc). Each aquarium can react differently.