testing the ph right out of the tap will give you a false reading,, aerate the water out of the tap for a couple hours then test..
you'll probably find that your ph is higher.. i would guess that your water is raising the ph, not your media
can you harvest rain water?
the ph in the system will drop on it's own, but that takes quite a while - my 110 gallon ft system took over 8 months for the ph to drop, and now i have to buffer it back up with shell grit..