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There are two main types of AP systems, coupled and decoupled. The coupled approach is widely used and is based on feeding the system known nutrient-input amounts/values. The support for plant growth and bacterial consumption (in the biofilter) typically come from commercial fish food and must be factored into system input requirements. These ratios are used to ensure that toxic waste products from fish effluent do not build up (due to an insufficient biofilter), excess nitrates do not occur (from not enough plants), and nitrate deficiencies do not develop (from an excess of plants). Recommended operating ratios for aquaponic systems will be covered in the Structure and Design section.
Given the wide range of growing conditions among fish, plants, and bacteria, coupled systems do not operate at the optimum values for either fish or plants. The ideal nutrient environment for fish would usually be nutritionally inadequate for most plants, and an ideal nutrient level for plants would be toxic to most fish. For this reason, decoupled systems are being explored, though their use is not widespread. In a decoupled aquaponic system, the RAS and hydroponic components are joined but operate as separate systems that can be controlled independently (Goddek et al. 2016, Pantanella 2013). Typically, water that feeds the hydroponic system does not enter back into the fish culture tanks after being filtered by the plants. Instead, water lost though transpiration and evaporation in the hydroponic unit is replaced with water from the RAS, which in turn is replaced with new water (Kloas et al. 2015). This setup offers greater control over the individual system and allows each to be operated at their optimal range. Disease treatment and nutrient deficiencies (or toxicities) are more easily managed, as well. Decoupled systems are not as well researched as coupled systems and require producers to have a higher level of expertise in hydroponics, plant nutrient management, and aquaculture system design.
Source: Janelle Hager, Leigh Ann Bright, Josh Dusci, James Tidwell. 2021. Kentucky State University. Aquaponics Production Manual: A Practical Handbook for Growers.