Although hydroponics is a soil free form of agriculture, it still requires that the plants receive nutrients.
Just about anything that is sterile, of a neutral Ph, reasonably dense but formed of large particles or pellets can be used as a growing medium. It should be easy to work with and anchor the plants without restricting the flow of water, nutrients and oxygen around the plant roots.
There are many commercially produced cubes and they are ideal for growing plants from seed. They are made from the compressed fibers of rock-wool, coconut or coir, peat, wood, straw and just about anything else that can be sterilized. Once the seedling has grown the whole cube can be inserted into your hydroponics system. If using these cubes, be very careful about over watering in the early stages because of the danger of molds developing.
Coconut fiber is rapidly becoming the most popular of all mediums. It is 100% natural and has excellent water retention properties while helping to maintain good circulation of oxygen around the roots. The natural fibers created by grinding coconut shells also have an anti fungal property. When mixed with clay pellets the density is ideal.
Perlite is familiar to many gardeners as it is used to add drainage properties to poor soil. Because perlite has little water retaining ability it should be used with other products or you will spend most of your time worrying about the lack of water being delivered to your plants. Perlite dust is unhealthy to inhale, so use a respirator or dust filter when handling the dry product.
Vermiculite is almost the perfect mix for perlite because it retains up to three times its own weight in water. Used by itself, it would quickly smother the life out of your plants. A 50/50 mix of perlite and vermiculite is almost the perfect growth medium. When handling vermiculite you should use a respirator or face mask because the dust is not healthy to inhale. Vermiculite contains tiny asbestos fibers.
There are many commercially produced soilless mixes and they all are suitable for hydroponic use. The natural and mineral ingredients of soilless mixes enable them to claim organic status. They are usually designed to offer good drainage