Coupled or De-coupled, That is the Question

Aquaponics systems are all based on a connected relationship between fish and plants. But what that relationship is and how it is systemically designed are the source of great debate in the aquaponics community. The two sides of the debate are coupled and decoupled, but the reality is most people fall into a middle ground somewhere between the two.

Coupled systems

Those who choose the truly coupled systems often refer to themselves as aquaponics purists. Coupled systems can work fine for those who want to primarily produce plants and aren’t particularly concerned with the quantity of fish they raise. The water from the fish carries the fish waste from their feces and the ammonia from their breathing and routes it through plant media beds which act as filtration units to purify the water as it makes its way back the fish. The plants feed on the nitrogen produced by the bio-purification process in the media beds. So the fish and plants support each other. This process sounds great, but there are some serious drawbacks and limitations.

First your fish stock is limited by the amount of media bed you have in the system. Media beds can work fine, but they have some limitations. And the reality is they have a very limited bio-filtration capacity. If you run too much fish waste through your media beds, you can clog or crash your system, running the risk of poisoning both your plants and your fish. This is why coupled aquaponics advocates spend a lot of time talking about and working on balancing their systems, and why they all talk about the limitations of growing fish in aquaponic systems. A truly coupled aquaponic system can only carry a very limited amount of fish compared to the amount of plants being grown. But it can be a good option for those who are mostly interested in ornamental fish that exist primarily for their fertilizing capacity.

Decoupled Systems

In a truly decoupled system, the plants and fish complement each other, but they aren’t actually dependent on each other. Decoupled systems evolved from the aquaculture world, where the primary goal was the production of fish for market, and where the plant growth was developed as an additional or bonus profit stream. However as it evolved, those experimenting with aquaponics figured out that they could use the fish waste to produce amazing plant food, and further that the plant production and profitability could actually exceed that of the fish.

Essentially a decoupled system runs the fish water through a solids filter to remove the feces waste, and then through a biofilter to convert the ammonia into nitrate which is essentially harmless for the fish. In a truly decouple system, the solids waste is then flushed into a digester or nutrifier where through a process utilizing aerobic and anerobic digestion the waste is broken down into a wonderful plant food that is circulated through the grow beds. But once the water and waste leaves the fish system and goes into the plant system, it never returns. So the fish waste is used to feed the plants, but the plants really offer nothing to the fish. If you have issues with either side of the equation, fish or plants, it is much easier to deal with in a decoupled system, because they are not directly influencing each other. If you are starting out with a new fish stock that is too small to provide the necessary nutrients to your plants, you can supplement your plants with a natural organic plant fertilizer without having to worry about what it will do to your fish.

Hybrid Systems

While both sides will argue for the benefits of their system designs, hybrid systems offer the greatest benefits drawing from both sides. A hybrid system can be coupled to produce the most natural, efficient and cost effective bio-system. However, they also utilize filtration systems and nutrifiers or digesters to allow for more intensive fish production. The expense of setting up a hybrid system is comparable to a de-coupled system, and is greater than a simple coupled system, as it requires additional filters, a two part sump system, and additional plumbing. However the long term benefits of better management tools and increased fish production make hybrid systems the most cost effective options, and increasingly aquaponics growers are incorporating various elements of de-coupled systems in order to better manage their systems.

These are out favorite hybrid aquaponic systems – check them out!