CaribSea South Seas Reef Rock – Arches (20 lb) – Large natural aragonite rock formations pre-shaped into arching structures, ideal for instant aquascaping of reef tanks. Each piece forms a graceful arch that can support corals or serve as a swim-through tunnel for fish. Made of ancient reef material, these arches are highly porous to aid biofiltration and are completely free of harmful impurities. They allow you to easily construct an intricate cave network or coral bommie in your aquarium, creating a realistic ocean reef appearance with minimal effort.