Get a cuttlebone, they sell them at pet stores with the bird supplies. Just break it and put a piece in your filter or under some decorations. It will add calcium to the water. Or, the next time you cook some eggs, clean the shells very very well, pull the membrane off them, then bake them for a little while to kill any bacteria, then crush it and add it to your substrate. Or you could use the supplements available in the fish store for marine aquariums. Just get a calcium test kit too.