Unless you have something to buffer the water, pH is almost always going down in the aquarium. Carbonic acid is produced by the reaction of CO2 and water (and our blood interestingly, known as the Bohr effect in medicine), and this is why many people time their pressurized CO2 systems around their lighting (light on is when plants utilize it). But even when using off-gassed water, most biological processes reduce pH in the aquarium. What is your goal pH in these tanks? Crushed coral, oyster grit (same thing), seashells, rocks containing lime, and all manner of things can be added to keep it from dropping as quickly if that is what you are after.
Nothing good happens fast in an ecosystem.