A large variety of filter media are used in natural swimming pools. The most important aspects of filter media are the surface area available for bacterial attachment, and the chemical makeup 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 occurring deposits or manufacturing plants. However, if budget allows, 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 adsorption and can thus be highly beneficial to the system. Another consideration is the crystalline 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.