I read that you can just tie down the cuttlebone and leave the soft side up. I have ramshorns and stuck in one of those 7-day feeder cubes (chopped up). The don't eat it but I'm sure as it disintegrates the calcium is released in the water.
When I had cuttle bone for my turtles years ago, it would last forever in the water. Crack yours in half and tie it down and you should be good for a while.