If the above-listed media are unavailable, it is possible to use other media. Alternatives
include: river-bed gravel, which is usually limestone but can have a low surface area to
volume ratio depending on the granulometry; pumice (also rockwool), a white/grey
volcanic material also popularly used as growing medium in hydroponics; recycled
plastic, although plastic floats and needs to be held submerged with a layer of gravel on
top; or organic substrates such as coconut fibre, sawdust, peat moss or rice hull, which
are often inexpensive but risk becoming anoxic, deteriorating over time and clogging
the system. However, organic substrate can be used for a time within aquaponics, and
once it begins to deteriorate, the media can be removed from the system, composted,
and used as a valuable soil addition for soil crops. Table 4.1 summarizes the major
characteristics for all the growing media mentioned above.