Made with materials for a simple finish. We carefully select the grain of the white wood with fine foam. Because it is made of natural wood, there is a solid material that is hard to foam. After drying, use again will cause bubbles to appear. When used with a thin skimmer, dirty sea water is trapped from the foaming surface of the woodstone, and the saltwater cleaner than the unfoamed surface is drained into the aquarium. No glue, bio-safe. Easy to clean as it is replaced.