ML wrote:
I have a 20g fresh water tank that is 3+ years old.
Our water is naturally soft and I have problems keeping the PH up. It is a
community tank and I would like to keep the PH around 7.0. It is currently
about 6.6.

Add material made of limestone, e.g. seashells, chalk (not black board
chalk, which is something completely different, but the stone, the
'white cliffs of dover' chalk) or limestone. Marble reacts too slow to
be of much use though.

Limestone etc will buffer your pH to 7.5 - 7.7, which is exactly what
you want.