Grow media in hydroponics, aquaponics, or any other soil-less system is present primarily to give roots something to hold onto and remain upright, but be easily separated from roots when the produce is ready to harvest. The ideal grow media should be smooth and moderately lightweight, pH neutral, non-degradable, and offer plenty of surface area. Why surface area, you ask? A critical element of the aquaponics nutrient cycle is the presence of certain types of bacteria that break down fish waste and convert it to nutrients accessible by the plants. Bacteria are microscopic, and millions are required, so they need plenty of space to grow and multiply. There will be some in the biofilter, of course, but ideally, far more will take up residence in tiny nooks and crannies throughout the system. So grow media that is formed of a material that is highly porous on a microscopic scale will permit bacteria colonization while resisting intrusion of roots and other organic bits.
Neutral pH is necessary to keep your water chemistry at healthy levels. If you’re constantly fighting the effect of a grow medium like limestone, life will not be pleasant. Your media should be lightweight for easy handling, but not light enough to float or be carried around by water flow, and it should not decompose or break down. This is critical. Decomposition will throw your nutrient cycle out of balance and grow media that breaks down could clog or damage your equipment and will need to be replaced periodically. Two excellent options for grow media include Lightweight Expanded Clay Aggregate (LECA), which are essentially balls of clay that have been heated at extremely high temperature. They check off all the requirements of an ideal grow medium, and although they have a higher price tag than some, they’re reusable virtually forever, so it’s a one-and-done investment.
