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A large variety of filter media are used. The most important aspects of filter media are the surface area available for bacterial attachment, and the chemical makeaup of the material.
Media such as expanded clays, and pumice have the advantage of being porous and therefore can host a large number of bacteria. But they are expensive, especially in South Africa, where we have no locally occuring deposits or manufacturing plants. However, if budget allows, then using porous materials can be worthwhile.
If crushed stone is used, it is vital to understand the chemical makeup of the rock. Rocks can can contain a lot of nutrients, which will continuously pollute the system, and can also be toxic to life. On the other hand, some rock has the perfect mineral chemistry to absorb certain nutrients through the process of chemical adsorbtion and can thus be highly beneficial to the system. Another consideration is the chrystaline structure of the rock, and how it breaks. One wants to select stones that break into sharp angles so that there are gaps between the stone for biofilm to grow into.
EcoPools sells the right type of mineral filtration medium, which has been chemically analysed and selected for nutrient absorbtion. The medium has been treated and cleaned and is ready to be installed.