My ph is 6.0 ( I have never had a ph too low - never dealt with it before)

I normally keep it 6.6

I know that this is not dangerous but I do want it to come up. I obvoiusly do not want it to spike and I would like to raise it w/o chemicals if possible.

I know that a partial water change will be the obvious way to do this (tap is 7.4).
I did a 25% change(with gravel cleaning) 2 days ago and it went up to 6.3/6.4 (OK fine) but then this morning it was back down to 6.0 again (?????).

How soon can another water change be performed?
Will a few seashells raise the ph?
should I temporarily remove one or both of my driftwoods?

Any thoughts on dealing with this. It is not an emergency, but I like to be precise and I am learning lately to do things as naturally(minimize chamical additives) as possible.

Thanks for any ideas and advice!
IP