You said nothing about fixing it in your original question - I assumed you were just asking a "why" is this happening question.
You can't fix anything until you know for sure what is the root cause of the problem - unless you are one that likes to shoot from the hip with a shotgun, hoping to hit your target.
We have little if any control on the water coming out of our taps, aside from installing and using an RO or RO/DI unit, or buying water from another source.
If you choose to do neither - then I'm afraid you're limited to your "choices" to correct it.
Adding marine buffers, using different freshwater, selecting a different saltmix, reducing your fish load and increasing your water circulation for better CO2 gas equalibrium may be your only hope. Too much CO2 in your tank can cause a significant drop in pH as well.
