[garfieldnfish]

You can also use eggs shells. After eating the yolks, lol, you can put the shells in some tank water for a few hours or days. That softens them up. Then you can crush them easily, so there won't be any sharp edges where the snails or shrimp could hurt themselves. Then either add some of the water or the shells themselves to the tank. Cheaper then buying the liquid stuff and everyone uses eggs now and then.
I do also use the liquid calcium from Kent and feed mine crab cuisine, rich in calcium and the snails love it. I have very soft water so I overdo the calcium supplement a little but I got my apple snails to breed several times already and ended up with more snails then I wanted.