Hydroton is a clay aggregate that is manufactured exclusively at high-tech kilns in Germany and is used as a soil replacement in hydroponics. Composed of shale that is pelletized and fired, Hydroton holds water extremely well and transmits it effectively. The pellets have an attractive, natural appearance and make an excellent drainage layer in the vivarium. 2 gallon of Hydroton is enough to add a 2" layer to the bottom of a standard 10 gallon tank.