Underwater Galleries Aquarium Rock Cave " Moon Rock" Glow-in-the-Dark Ceramic Cave is an ideal home for any aquatic pets! 4" wide and 2" tall: many fish, including Bettas, like to tuck themselves into this little cave to feel safe. Underwater Galleries Aquarium Rock Cave " Moon Rocks" Glow-in-the-Dark Ceramic Cave has a large main cave opening, a " skylight" opening to bring light to the interior of the cave, and a "backdoor" to allow a smaller fish to swim away if a larger fish enters. Thanks to Underwater Galleries special proprietary glaze, which encapsulates rare earth elements which absorb and release light energy, the Moon Rock glows in the dark. And it does not just glow it glows a beautiful MoonBeam Blue! This soft blue color create the illusion of having a glowing moon rock in your tank! To get the best glow; the aquarium should have a good light above the tank; and remember: the closer the light the better glow!Made of 100 percent eco-friendly clay, fired to high temperatures to create this high quality ceramic. Safe for all tanks. Underwater Galleries " Moon Rock"a night light for your fish!.