Bio-filtration eats up the KH which supports (buffers) your pH; when the KH drops, so does the pH.
The easiest way to keep your KH (and therefore your pH) at baseline (tap/well) levels is to do sufficient water changes. That way, the readings won't drop and therefore won't need to be snapped back into line.
This, of course, assumes that your tap/well water is of satisfactory quality/quantity in the first place. If that's not the case, then baking soda can be used to boost the KH.
So like Kathy asked, what are your baseline (tap/well) readings for pH, KH & GH?