shells in african tank

I just setup my first tank for africans. No fish yet...I have playsand substrate. I'm wondering if shells do an equal job of raising hardness and pH as does crushed coral. Are all shells made of calcium carbonate?

I also have a small bag of crushed coral in the filter and a small bag hidden behind some rocks under the filter intake. Tank is a 20long. Would this be adequate for attempting rift lake conditions?

Thanks