DG22

My water has a ph 6.8, kh 40 ppm, and gh 300 ppm. I tested my water this afternoon to find that the kh was nonexistent. the ph slipped down to 6.4. I went to both towns nearby to find out no pet stores have crushed coral. As I was looking around on the internet while in town I found "oyster shell poultry treat" at the tractor supply. The ingredients say "oyster shell and coral calcium, clam-coquina like shell." Is this pretty much the same thing as crushed coral for aquariums?