This listing is for 20 slow dissolving calcium blocks. Can be used in any aquarium where extra calcium is needed, they will eat some and the rest dissolves into the water column to be absorbed. It also helps to raise ph. 1 tab per 0-10 gallons ( takes about a week to dissolve), garunteed it will not cloud your water, and its more convenient than cuttlebone because it sinks and stays put. Made with food grade calcium sulfate and food grade calcium carbonate without any D3 or extra fillers to help keep your snails shells looking healthy and in good condition. New shapes and sizes coming soon. Free shipping.