Which substrate should you choose for aquaponics ?

In an aquaponic system, the substrate plays a much more complex role than in traditional gardening. In a traditional vegetable garden, the soil simply supports the roots and provides nutrients. In aquaponics, this role is multiplied, as the substrate becomes one of the pillars of the ecosystem. It first acts as a solid anchor for the plants : their roots must develop there without constraints, to ensure good access to water and nutrients. But it is not just a simple physical support. It is also a place where intense biological activity develops.

Nitrifying bacteria, invisible to the naked eye but essential to the nitrogen cycle, find the substrate an ideal surface for growth. They transform the nitrogenous waste produced by fish into compounds that can be assimilated by plants, allowing the ecosystem to function in a closed loop. A substrate that is too compact or chemically unstable can compromise this process, leading to harmful imbalances for both fish and crops. This is why it is crucial to choose a material that is porous, durable, and neutral, ensuring good root oxygenation, optimal biological filtration, and water chemical stability.

Over time, several materials have established themselves as benchmarks in aquaponics.

Expanded clay remains the preferred choice for enthusiasts. Its lightweight, porous beads provide remarkable aeration and an ideal colonization surface for bacteria. Their neutral pH and durability make them a reliable medium, although their initial cost can be prohibitive for large volumes.

Washed gravel, often chosen for its more affordable price, is another attractive option. Stable and durable, it makes a good biological support, provided you ensure that it does not contain limestone or metals that could affect water quality. Its major drawback remains its weight, which complicates the installation and maintenance of systems.

Pozzolana, derived from volcanic rocks, is another popular substrate. Lightweight, porous, and inexpensive, it provides an excellent exchange surface for bacteria. Its major drawback is that it tends to clog very easily, and cleaning it is not at all easy.

Finally, coconut fiber represents a more ecological and organic alternative. Easy to handle, it retains water well and promotes seedling growth. However, it degrades quickly and must be replaced regularly, making it an interesting supplement but rarely a primary substrate. These materials must be chosen based on budget, space, and the crops planned.

Sphagnum moss is sometimes used in aquaponics for its excellent water retention capacity, making it an attractive substrate for seedlings. However, it clogs easily, making water circulation difficult. Its cleaning is complicated, making it impractical in the long term. Furthermore, importing it from distant regions raises ecological concerns, limiting its usefulness as a primary substrate.

While some materials are true allies in aquaponics, others represent pitfalls to avoid. Sand, for example, seems attractive because it's inexpensive and easy to find, but its accumulation of waste quickly prevents proper water circulation and deprives roots of oxygen, leading to rotting problems. Conventional soil and potting mix also pose a problem. They release substances that unbalance the ecosystem, attract parasites, and compromise the health of both fish and plants.

Limestone gravel, which is widespread in some regions, poses another danger. By releasing calcium carbonate, it alters the pH of the water, gradually alkalizing it. This chemical imbalance weakens the entire system, affecting the plants' ability to absorb nutrients and compromising the health of fish. Finally, chemically treated artificial substrates, such as certain polished glass or plastic beads, present a long-term risk of releasing toxic substances into the water. It is therefore better to turn to proven, stable, and neutral media to ensure the safety and longevity of the system.

The choice of substrate is not simply a matter of taste; it depends above all on the objectives and constraints of each project. Budget remains a determining factor : expanded clay is ideal, but its higher price sometimes limits its use, while washed gravel is an economical compromise. The type of cultivation envisaged also plays a major role. A deep growing bed requires a stable and durable substrate, while floating baskets or vertical towers require lighter materials better suited to this format. Maintenance frequency must also be taken into account. Some substrates are easy to clean and allow for more comfortable monitoring, while others, heavier, complicate handling and require more work.

In a region where the water is already hard, it is also not recommended to use a substrate that could further alter the pH. In this case, opt for a neutral material. Finally, it may be worth favoring local resources. Not only does this reduce costs, but it also limits the carbon footprint associated with transportation. In short, the right substrate is one that meets your production objectives, your financial means, and your environmental constraints.

Over time, the substrate becomes loaded with organic deposits, which can slow water circulation, reduce root oxygenation, and disrupt bacterial colonization. Therefore, its maintenance is essential to maintain the balance of the system.

Sometimes this requires complete disassembly, but in general, maintenance is mostly based on simple, regular actions. This involves observing the water flow, preventing the substrate from compacting, removing excess material, and, if necessary, partially rinsing with clean, non-chlorinated water. The goal is not to make the substrate sterile, but to limit clogging while keeping the precious nitrifying bacteria colonies alive. Some materials, such as coconut fiber, eventually degrade and must be replaced regularly, while others, such as expanded clay or pozzolan, can last for years provided they are cleaned from time to time and reused. The key lies in a delicate balance : intervening enough to keep the substrate effective, but without ever disrupting the ecosystem that has settled there.