Clay pebbles are a popular hydroponic medium around the world. Manufactured in Italy - they are derived from a renewable and plenitful source (clay); it is considered an ecologically sustainable medium. The clay is formed into pellets and then fired in rotary kilns at 1200°C. This causes the clay to expand inside (like popcorn) and to become porous - high surface area for colonisation by beneficial bacteria. FishPlant Clay Pebbles are light in weight, do not compact and are completely reusable - they can be cleaned and sterilised after use. They are also inert and pH neutral and contain no nutrients. They provide good oxygen levels around the root zone because the pebbles drain freely and do not hold excessive water.