I'm still struggling to get my pond's Hardness up where it needs to be. Thru researching possible corrective actions, such as gypsum, I came across the idea of adding cruched oyster shell to increase calcium carbonate levels.

Has anyone ever tried this, and if so, what were your results?

I ask primarily from a cost standpoint, as 50# of oyster shell is about the same price as 10# of gypsum.

Dolmitic Ag lime has both calcium and magnesium. Pelletized Dolomite is a little pricey, but very easy to spread and effective. Pulverized limestone is cheaper and also very easy to spread and effective....Count on needing 5 ton per surface acre...more if using gypsum.

Oyster shell is comparatively very course, so it will not dissolve very fast, but can never hurt.

I added 500# of aglime to my little pond a few months back. It raised my Alk a bit, so I probably need another 500#. My Ph is relatively stable, never over 8 or under 7 now. However, my hardness is still nill. Thats my reason for looking for something besides aglime.