These large solid marble pebbles can be used to decorate or cover the ceramic media without obstructing the flow of water into the filter. Available in black chosen for its Feng Shui properties that promote the banishing of negative energy. Once in water the color of the pebbles deepens and shines. They are made from marble which is a solid stone and won’t cloud the water. Rinse before using to remove dust created in transit. Suitable for all aquariums.