Many types of media and combinations of media may be used to grow plants hydroponically.  Choice is largely dependent upon availability, the type of system in use and the plants to be grown.

Perlite:    Perlite is a volcanic rock, which when heated in excess of 1000 degrees Celsius expands into lightweight particles.  In hydroponics approximately 3mm particle size should be used.  The medium is inert and sterile and is used as a lightweight support medium.  It does not have a high capacity for water retention and has no nutrient hold capacity.  Perlite can be used on its own or with vermiculite, depending upon specific plant needs.  The medium mast be pre-dampened before use.  Avoid inhalation of the dust.

Vermiculite:    An exfoliated mica, vermiculite is light and has a very high capacity for water retention.  Because of its flaky structure, vermiculite eventually breaks down and requires replacement every 2 to 4 years.
Warning: Vermiculture breaks down and this can cause problem to pump based irrigation systems.

Mixes:    A 50/50 mix of perlite/vermiculite makes and ideal growing medium.  It is advisable not to mix sand, scoria or crushed granite with perlite or vermiculite, as they will separate when wet due to difference in densities.

Rockwool:    Horticultural rockwool consists of a mat of long, fine fibres, spun from molten natural rock.  It is inorganic and inert, very light when dry, sterile, is not biodegradable and has a very high water and air holding capacity.  It is sold in Australia under the name of Grow-wool.

Hydroton Clayballs:    These clay balls are made from expanded clay and provide fantastic water retention and aeration which is a big requirement for any hydroponic gardening.