## 5 Conclusions  
We got support for our first hypothesis about the onset of nitrification process, as it was faster in the aquaponics treatment than in RAS. The second hypothesis regarding the concentration of off-flavor compounds in fish flesh was supported only partly, as the GSM concentration was lower in rainbow trout flesh grown in aquaponics but the concentration of MIB was at a similar level in fish reared in aquaponics and RAS. The third hypothesis about the concentration of off-flavor compounds being higher in the plants grown in aquaponics than in hydroponics was supported partly, as the concentration of GSM was higher in the roots and MIB was higher in the shoots of spinach grown in aquaponics than in hydroponics. In the fourth hypothesis we assumed similar growth for the plants and fish. Spinach grew equally well in both aquaponics and hydroponics treatments, but the aquaponics system was better in terms of fish growth with improved FCR, likely because of the better water quality in the aquaponic system.  
**Supplementary Materials:** The following supporting information can be downloaded at: [[https://www.mdpi.com/article/10.3390/w14091447/s1](https://www.mdpi.com/article/10.3390/w14091447/s1)]([https://www.mdpi.com/article/10.3390/w14091447/s1](https://www.mdpi.com/article/10.3390/w14091447/s1)), Table S1: Limits of detection (LOD), limits of quantification (LQO) for micronutrients (mg/kg dm) Fe, Cu, Mn, Zn, B and macronutrients (g/kg dm) N, P, K, Ca, Mg, S for ICP-OES and ICP-OES methods.; Table S2: Limits of detection (LOD), limits of quantification (LOQ), and linearity (R\({}^{2}\)) of selected standard solutions (I-100 mg/L) for HPIC analysis of anions.  
**Author Contributions:** Conceptualization, F.A., J.P. and P.L.-L.; methodology, F.A., J.P. and P.L.-L.; validation, F.A. and P.L.-L.; formal analysis, F.A. and P.L.-L.; investigation, F.A. and P.L.-L.; resources, F.A. and P.L.-L.; data curation, F.A. and P.L.-L.; writing--original draft preparation, F.A.; writing--review and editing, F.A., J.P. and P.L.-L.; visualization, F.A.; supervision, J.P. and P.L.-L.; project administration, F.A.; funding acquisition, F.A. and P.L.-L. All authors have read and agreed to the published version of the manuscript.  
**Funding:** This research was funded by Maa- ja Vesitekniikan Tuki ry, Niemi-Foundation, and European Maritime and Fisheries Fund.  
**Institutional Review Board Statement:** The fish used in this study did not experience at any moment pain, distress or suffering that would be comparable to the introduction of a needle into the body. Therefore no experimental animal permit was needed according to the Directive 2010/63/EU of the European Parliament and of the Council on the protection of animals used for scientific purposes. The laboratory has been accepted by the Regional State Administrative Agency on 27 March 2018 for keeping fishes for experimental purposes (permission number ESAVI/5100/04.10.05/2018).  
**Informed Consent Statement:** Not applicable.  
Data Availability Statement: Not applicable.  
**Acknowledgments:** We would like to thank Sisa-Suomen kalatalousyrhma and JAMK University of applied Sciences for covering the cost of aquaponics facility construction, facility management and providing facility for the experimentation and all kind of assistance during the project. We will extend our thanks to [PERSON] for transporting the fish, [PERSON] and [PERSON] for their support and encouragement throughout the project.  
**Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.