## 1 Introduction  
Partly due to the tightened demands for environmental permissions, especially in the land-based aquaculture, recirculating aquaculture systems (RAS) are gaining popularity in producing fish for human consumption. The main advantage of RAS is highly decreased water use compared to traditional flow-through systems. Consequently, the nutrients released by the cultured animals are highly concentrated in the limited amount of effluent, which can offer cost-efficient opportunities for nutrient reuse and wastewater treatment [1]. In RAS, the maintenance of the microbial environment in biofilters is essential because the microbes responsible for nitrification convert harmful ammonia excreted by fish, first to nitrite and then to nitrate [2]. Exposure of fish to even low concentrations of ammonia and nitrite can be harmful and affect the fish welfare and survival, while nitrate is a rather safe compound for the fish at concentrations <100 mg/L [3]. The start-up of the nitrification process using intact biofilter media can take up to two months [2], after which the levels of ammonia and nitrite should remain at levels that are safe for fish [1]. Several studies have been conducted to increase the efficiency and speed up the onset of nitrification in RAS [2, 4, 5]. For example, the nitrification efficiency in RAS has been studied by investigating the biofilter configuration and relationship between the heterotrophic and nitriifying bacteria, nitrification efficiency of the submerged biological filter, total ammonia nitrogen (TAN) concentrations and varying C/N ratios [4], biofilter media types and their effects on the efficiency of trickling filters [6; 7] and the effects of the design of the biofilter on the oxidation of ammonia [2]. Hydroonics refers to the soilless cultivation of plants where the nutrients for the plant's growth are provided in a solution [8], and the plants get the nutrients from the water instead of soil [9]. Hydroonics is an efficient method for producing vegetables with minimal water and space [10; 11]. Aquaponics refers to a system where RAS and hydroponics have been combined, and the RAS effluent with concentrated nutrients is utilized to grow plants [12; 13]. The ammonia excreted by the fish is converted in the biofilter to nitrate, which is easily absorbed by the plants [14]. The fish feed contains macro- and micronutrients essential for fish growth but that are also important for the plants [15]. While absorbing the nutrients from the RAS wastewater, the plants also clean the water from compounds potentially harmful to the fish due to low water exchange [15]. However, plants differ in their demand for nutrients, and their availability in RAS effluent may not be enough for all plant species. To cope with this situation and provide enough nutrients for the plant's growth, some nutrients can be provided in a solution [12]. Aquaponics has been regarded as a sustainable and environmentally-friendly method for producing plants and fish [12; 13]. In addition, it supports the idea of a circular economy as the wastes produced by fish are turned into a resource for the plants. The presence of bacteria like Cyanobacteria, Actinomycetes, and Myxobacteria in RAS can produce off-flavor compounds geosmin (GSM) and 2-methylisoborneol (MIB) [16; 17; 18] which easily accumulate in fish flesh and cause earthy and musty flavor. GSM and MIB are semi-volatile terpenoid compounds that accumulate in the lipid-rich tissues of fish. The main route of uptake is through the gills, but also via the skin and gastrointestinal tract, and the uptake proceeds fast, typically within hours [18; 19; 20]. The concentrations of GSM and MIB in fish flesh seek equilibrium with their concentrations in water. However, factors such as water temperature and flow rate, fish age, size, and species, along with the exposure time, have been shown to affect their concentrations [18; 19; 21; 22]. The removal of the off-flavor compounds from fish flesh is essential for it to be marketable.