How long will it last? This was my original plan as I can fit it in my filter but I was scared I would have to replace it a lot. Currently I am using rocks to do it but I would like to replace them as they don’t go along with my scape.toran said:You can put cuttle bone in the tank for calcium. It floats at first but you can put it under a rock or something for awhile. It doesn't cost much and you can break it in smaller pieces.