How much oyster shell?

I just bought a bag of crushed oyster shell to add to my pond. Been
having alot of trouble with acid rain and already acid tap water- which
I have to add every few days due to water loss/evaporation. Anyway,
I've been searching the forum on the subject of adding lime or oyster
shell and could only find the oyster shell. My question is, how much
should I add? My pond is roughly 2200-2600 gallons (misread the meter
when filling the pond, thus the estimate on the gallonage). I have been
adding baking soda, which did the trick until we got some rain and had
to "top-off" the pond again. Checked the water this morning and the PH
was 7.2 and the KH was around 80. It was 8.4 and 120, respectively. So,
Im hoping to add the oyster shell to have something in the pond that
will disolve as needed to help me keep up with the shifting water
quality. Just need to know how much I should add.