How Do Fish Provide What the Plants Need?

There’s a misconception that fish can somehow feed on the plants to sustain themselves, but commercial-sized aquaponics systems rely on inputs in the form of fish feed. That fish feed creates the nutrients the plants need, both when decomposing after being missed by fish and by turning into fish waste products. Fish release both liquid and solid waste, both of which are high in ammonia. While ammonia compounds are often used as fertilizer in soil-based agriculture, they’re too harsh for water-based systems like aquaponics. So biofilters and treatment tanks are used to convert that ammonia into nitrites, which eventually turn into nitrates after further processing by beneficial bacteria.

When this treated water is finally flushed down to the trenches or tubes containing the plants, it’s rich with the forms of nitrogen, phosphorous and potassium they can absorb best. If the plants were in direct contact with the fish waste in the original tanks, they’d receive too much nitrogen from the raw ammonia and mostly die off or fail to grow. Relying on the natural processes offered by bacteria, water movement and air exposure, results in a conversion from ammonia to nitrates. As the waste breaks down, it also releases plenty of the secondary nutrients plants need like magnesium, calcium and sulfur. Any fish that happen to die and start to decompose in the tank will also contribute these trace nutrients to the water supply, although they will also produce too much nitrogen and toxic compounds if not removed promptly.

While the solid waste that breaks down in the fish troughs and tanks is also a potent source of fertilizer, most of it doesn’t make it out of those tanks. This leads to the need for extensive cleaning among cohorts of fish so the water quality isn’t affected by the breakdown of too many waste products. These solid remnants are valuable as a fertilizer source as well as for in-ground and container growing, so consider drying and selling them rather than just discarding the material, as it represents a potential source for secondary income.

What Conflicts Can Occur Between Fish and Plant Needs?

When designing an aquaponics system from scratch, rather than following a pre-made plan or design, you must pay careful attention to balancing the needs of both the fish and the plants. Choosing a fish species with a high stocking rate and high feed demand will result in a heavy nutrient load, so planting low-nutrient-preferring plants like leafy greens and radishes in the system will result in poor growth. Similarly, raising a warm water fish like catfish and dispensing that water to plants that need cool temperatures will only hurt the total efficiency of the system.

In most cases where more nutrients are needed by either plants or fish, there are plenty of options for controlling the exchange between the two halves to the system. For example, additional nutrient requirements for plants are easily dealt with, as even recirculating systems contain filters and surge tanks for removing excess nutrients before water is returned to the fish tanks. When the excessive levels of nutrients or high temperatures start with the fish in the system, adding an extra tank or two to facilitate cooling and to settle out suspended solids or dissolve volatile compounds can help the water. Some systems may require you to treat the water manually with pH balancing and ammonia-reducing chemicals each time you discharge a fresh flow through the system. If you’re having to put this much work into maintaining the right water quality for the plants, you likely need to redesign the system in some way to better align the needs of both plants and fish instead of keeping up with multiple treatments a day.