biOrb Feng Shui Pebble Pack Add color to the bottom of your aquarium by placing these polished marble pebbles on top of the ceramic media Compatible with Freshwater/Saltwater/Tropical aquarium set ups Midnight Black is the color associated with banishing negative energy. Promote the flow of positive energy throughout your home while giving your aquarium the 'finished look'. Pebbles deepen in color when placed in water.