A good hydroponic medium should support the plants in the system, not break down, absorb and retain moisture, be porous to allow air circulation and proper drainage, and protect plant roots from temperature extremes.
Different media are suitable for different crops.
It's much like rockwool, but it doesn't irritate the skin when dry. Also, it's biodegradable, so disposal doesn't pose a problem. Like perlite, clay pellets are porous and retain water and air well. Expanded clay is dusty, so it will need to be rinsed before it goes into your system. Perlite Perlite is lightweight, stable, and has good water-retention properties.