Introduction
In response to global environmental challenges such as climate change, the scarcity of freshwater, and soil degradation, there is growing interest in sustainable and resource-efficient agricultural systems. Among these, aquaponics, a recirculating system that integrates aquaculture and hydroponics, has emerged as a promising alternative to conventional farming methods (Goddek et al., 2019). A recirculating aquaponic system recycles the nutrient-rich effluent from fish tanks to support plant growth, thereby minimizing the use of synthetic fertilizers and reducing environmental discharge (Graber and Junge, 2009; Love et al., 2015).
Numerous studies have demonstrated the ecological and economic benefits of aquaponic systems, particularly in terms of water use efficiency, nutrient recycling, and reduced chemical inputs (Yep and Zheng, 2019; Goddek and Körner, 2019). However, the vast majority of research to date has concentrated on leafy vegetables such as lettuce and herbs, which require relatively modest nutrient inputs and shorter production cycles (Ferrarezi and Bailey, 2019; Nozzi et al., 2018). In contrast, fruiting crops, particularly cherry tomato plants, which exhibit prolonged reproductive cycles and high macronutrient demands, have received relatively limited empirical attention in aquaponic research compared to leafy vegetables.
Fruit vegetables are evaluated not only in terms of their biomass yield, but also on fruit quality indicators such as °Brix (sugar content), acidity, firmness, and skin color, all of which are highly sensitive to water quality, nutrient availability, and ionic composition (Wu and Kubota, 2008; Resh, 2012). While hydroponic systems allow for precise control over nutrient concentrations and composition, aquaponic systems often exhibit suboptimal levels of key macronutrients such as potassium (K+), calcium (Ca2+), and nitrate (NO3−), which are essential for proper fruit development (Delaide et al., 2017; Rakocy et al., 2012; Saha et al., 2016).
Recent studies have attempted to address these limitations by implementing advanced designs such as decoupled systems (Goddek and Körner, 2019), optimized feeding strategies (Duarte and Cerozi, 2024), and nutrient supplementation protocols (Delaide et al., 2016). For instance, Rezaei et al. (2025) demonstrated that cherry tomato yield in aquaponic systems can match or exceed that of hydroponic systems when improvements such as enhanced fish tank illumination or biofilter separation are applied; however, research in this area is still limited. Koi are omnivores with a varied diet that typically includes plant-based foods, such as algae, as well as commercial feeds with 25 to 32% protein (Watson et al., 2004). The nutrient-rich waste produced by koi, particularly in the form of ammonia, plays a vital role in providing nitrogen for plants (Roosta, 2014; Yep and Zheng, 2019). Ammonia excretion is a key feature of koi metabolism, and the conversion of ammonia to nitrate through nitrification processes is essential for the growth of plants in aquaponic systems (Mullins et al., 2016). Furthermore, koi are well-suited to systems with variable nutrient levels, and are the second most commonly used fish species in commercial aquaponics, after tilapia (Love et al., 2014; Yep and Zheng, 2019). These attributes—dietary diversity, efficient nutrient recycling, and adaptability—make koi an effective and sustainable choice for aquaponic systems, ensuring nutrient availability for plants while maintaining water quality.
Koi (Cyprinus carpio haematopterus)—a cold-tolerant ornamental species widely cultivated across East Asia—has garnered substantial interest among aquarists and breeders owing to its vivid pigmentation, distinctive color patterns, elegant body morphology, and swimming behavior (Andrian et al., 2024). Koi-based aquaponic systems may offer enhanced environmental stability in temperate regions due to the species' adaptability and feeding behavior.
Yet thus far, there has been virtually no empirical data on the performance of koi-based aquaponic systems in the production of fruiting crops such as cherry tomato plants. Therefore, the present study aims to evaluate the growth performance and fruit quality of cherry tomato plants (Lycopersicon esculentum Mill.) cultivated in a koi-based closed-loop aquaponic system, and to compare the results with those obtained in a conventional hydroponic system. By analyzing plant physiological parameters (e.g., height, leaf number, chlorophyll content, stem diameter) and fruit quality indices (e.g., acidity, sugar content), this study seeks to provide foundational data for the practical application of aquaponics in the cultivation of fruiting crops, particularly under temperate, ornamental-fish-based conditions.
Research Methods
The aquaponic system
The experiment was conducted in a greenhouse facility with a total area of 224 m2 (7 m in width × 32 m in length). The aquaponic system was designed as a recirculating aquaculture system (RAS) integrated with hydroponic planting beds. The water circulation and filtration system included a biofiltration unit equipped with a biohelix media filter (580 mm in diameter × 1,380 mm in height) to support nitrification processes. The aquaculture component consisted of a primary fish tank (3 m in diameter, 90 cm in depth) for housing koi (Cyprinus carpio haematopterus), a 292 L recirculating filtration system, a 200 L biohelix unit, a sump tank and a settling tank (1 m in diameter, 1 m in depth) for managing water volume and overflow control. For the plant cultivation component, nutrient film technique (NFT) hydroponic beds were installed. Single-layer channels are arranged in 16 rows, each row connected in two rows measuring 15 cm in width and 5 m in length. These grow beds were used to grow cherry tomato plants (Lycopersicon esculentum Mill.) under controlled conditions throughout the experimental period.
The experimental design was based on two separate systems: an aquaponic system (eight rows of channels treated with aquaponic fish effluent) and a hydroponic system (another eight rows treated with conventional hydroponic nutrient solutions). Each system was operated independently, with distinct water circulation and filtration. This setup ensured that the aquaponic and hydroponic systems were completely separated, thus preventing any interaction between the two treatment groups (Fig. 1).
Investigational setup
Plant growth and environmental conditions
Cherry tomato seedlings were initially germinated in rockwool cubes and transplanted 40 days after sowing. A total of 66 plants were used in the experiment, with 33 plants assigned to the nutrient solution (hydroponics with Yamazaki nutrient solution) group and 33 plants to the fish tank water (koi-based aquaponics) group. During the cultivation period, the electrical conductivity (EC) of the nutrient solution ranged from 0.4 to 2.0 dS·m−1, while the EC of the aquaponic water (fish tank effluent) ranged from 0.3 to 0.6 dS·m−1. Average weekly measurements of temperature and relative humidity inside the greenhouse were recorded to monitor environmental conditions (Fig. 2).
Fish rearing and water quality management
A population of 150 koi (Cyprinus carpio haematopterus) was maintained and provisioned with ~500 cc of 13 mm extruded pellets, administered six times per week. Calcium (200 g) and iron (20 g) were added to the aquaponic water twice during the experiment to keep the calcium concentration in the fish water above 128 mg·L−1 and the iron concentration above 2.0 mg·L−1 (Lattauschke, 2004). Water quality in the aquaponic fish tank was monitored weekly using an API® Freshwater Master Test Kit (5-in-1, Mars Fishcare, Chalfont, PA, USA). During the experimental period, pH was maintained at 5.5–6.5, GH at approximately 180 mg·L−1, KH within the range of 40–120 mg·L−1, nitrite (NO2−) at 0 mg·L−1, and nitrate (NO3−) levels between 40 and 80 mg·L−1.
Assessments of growth performance
Growth performance was evaluated through a total of 10 assessments, consisting of two weekly pre-transplant measurements and eight weekly post-transplant measurements. The following growth parameters were recorded: plant height, leaf count, leaf length, leaf width, and stem diameter (measured using a digital caliper; SD500–150PRO, Sincon, Korea). Additional physiological and yield-related indicators included fruit set count, chlorophyll content (measured using an SPAD meter; SPAD-502Plus, KONICA MINOLTA, Japan), and fruit quality parameters such as soluble sugar content and acidity (measured using a portable sugar-acid meter; SAM-706AC, GMK Product, Korea).
Data analysis
A repeated measures analysis of variance (RM-ANOVA) was conducted to examine the effects of time and treatment group on plant growth. The within-subject factor was time (representing weeks 1–10), and the between-subject factor was group (aquaponics and hydroponics). Prior to analysis, the assumptions of normality and sphericity were tested. Mauchly’s test indicated that the assumption of sphericity was violated (p < 0.05); therefore, the Greenhouse-Geisser correction was applied to adjust the degrees of freedom. The soluble solids content (°Brix) and acidity of harvested fruits, which were measured once after harvest, were analyzed using an independent samples t-test to compare the differences between the two treatment groups.
Statistical analyses were performed using SPSS software (version 25 for Windows; IBM Corp., Armonk, NY, USA). Mean values and standard deviations were calculated using Microsoft Excel (version 2021; Microsoft Corp., Redmond, WA, USA). A p-value of less than 0.05 was considered statistically significant.
Results and Discussion
This study investigated the comparative effects of a koi (Cyprinus carpio haematopterus)-based closed-loop aquaponic system and a conventional hydroponic system on the growth performance of cherry tomato plants (Lycopersicon esculentum Mill.). An RM-ANOVA was conducted to evaluate the effects of time and cultivation method on the growth characteristics of cherry tomato plants (Tables 1 and 2). Across all measured traits—including plant height, stem diameter, leaf development, chlorophyll content, and fruit set—time had a significant effect, reflecting continuous growth throughout the cultivation period. In addition, significant time × group interactions were observed for most parameters, indicating that the aquaponics and hydroponics groups exhibited different growth trajectories over time. While early vegetative traits such as leaf width and chlorophyll content tended to be higher in the aquaponics group during the mid-growth stage, the hydroponics group ultimately showed superior performance in reproductive traits, particularly fruit set. Detailed statistical values for each parameter are provided in Table 2. The independent samples t-test revealed that both the soluble solids content and acidity were significantly greater in the hydroponics group than in the aquaponics group (Table 3, p < .001). Overall, these findings indicate that all major growth parameters of cherry tomato plants changed significantly over time, and the aquaponics group exhibited distinct growth patterns compared with the hydroponics group, particularly as cultivation progressed.
To assess live weight of koi, six koi were randomly selected from the population before and after the experiment. The mean individual weight was determined to be about 2.5 kg. No statistically significant difference in live weight was observed before and after the experimental period (p > .05, data not shown).
During the experimental period, neither fish mortality nor the occurrence of plant pathological symptoms was observed. The initial advantage observed in aquaponic cultivation may be attributed to beneficial microbial activity and buffered root-zone conditions (Kasozi et al., 2021). Aquaponic systems have been found to support crop growth to a degree comparable to conventional hydroponics despite lower concentrations of nutrients such as potassium, calcium, and iron (Rakocy et al., 2006, Seawright et al., 1998), which is assumed to be attributable to the suspended organic solids present in aquaponics water (Goddek et al., 2015; Sonneveld et al., 2009).
Nevertheless, the subsequent decline in growth performance in aquaponics is likely due to insufficient concentrations of essential mineral ions, particularly potassium (K+), calcium (Ca2+), and nitrate (NO3−). The reduced reproductive growth observed in the aquaponic system compared with conventional hydroponics can be mechanistically explained by ionic imbalances in the root-zone environment in the present study. Insufficient concentrations of key ions such as potassium (K+), calcium (Ca2+), and nitrate (NO3−) likely disrupted osmotic equilibrium, impairing water uptake and translocation within plant tissues. Potassium plays a central role in regulating osmotic potential and turgor-driven cell expansion, and potassium deficiency has been associated with decreased assimilate transport to reproductive organs (Hu and Schmidhalter, 2005; Almeselmani et al., 2009). Similarly, calcium deficiency interferes with membrane integrity and pollen tube growth, ultimately reducing fruit set and quality (White and Broadley, 2003). As noted by Xu et al. (2012), nitrate deficiency can suppress reproductive growth in fruit-bearing crops by disrupting the balance between carbon and nitrogen assimilation, reducing cytokinin-mediated reproductive signaling, and limiting osmotic regulation. Therefore, the combined deficiency of these ions in aquaponic nutrient solutions may have induced osmotic stress, restricting assimilate allocation to developing fruits and leading to reduced reproductive performance.
In the present study, measured ion concentrations in the aquaponic solution were in the range of 40–80 mg·L−1 (NO3−), significantly lower than standard hydroponic ranges (150–250 mg·L−1 for NO3−) (Lattauschke, 2004; Sonneveld and Straver, 1999). Recent studies further emphasize that maintaining a potassium ion concentration of 300–350 mg·L−1 in the nutrient solution positively affects water and nutrient uptake capacity, ionic balance among essential nutrients, and overall yield (Song et al., 2025; El-Nemr et al., 2012). Delaide et al. (2017) reported that aquaponics maintained a very low K+ concentration (< 10 mg·L−1) compared to the recommended 210 to 430 mg·L−1 in standard hydroponic solutions. These deficiencies are particularly critical for fruiting crops such as cherry tomato plants, as calcium and potassium play essential roles in fruit development, flavor, storability, and the prevention of physiological disorders such as blossom-end rot (Park and Kim, 2017).
The findings of this study are consistent with those of previous research and confirm that while aquaponic systems may offer physiological benefits during early growth, nutrient limitations significantly hinder reproductive performance in fruit-bearing crops. To address these limitations, several studies have proposed targeted supplementation protocols—such as potassium supplementation at 150 mg·L−1 (Harika et al., 2024)—as well as the implementation of decoupled aquaponic loops that allow independent nutrient management in the plant section without disrupting fish welfare (Goddek and Körner, 2019). Aslanidou et al. (2024) demonstrated that such strategies improved tomato yield by over 55%, while leaf count and plant height were also greater in decoupled aquaponic systems, reinforcing the practical value of nutrient balancing: however, these systems are often limited by higher initial investment, greater system complexity, and increased management requirements compared to conventional coupled designs, highlighting the need for further research.
Beyond the deficiencies in potassium, calcium, and nitrate discussed above, other factors related to micronutrient availability, pH buffering, and microbial biofilm activity may have also influenced plant growth performance. In aquaponic environments, these secondary factors can interact with macronutrient dynamics, further shaping both physiological responses and reproductive development. Micronutrient deficiencies, particularly of Fe, Zn, and B, may have further limited reproductive development in cherry tomato plants by impairing pollen viability, fruit set, and assimilate partitioning. As noted by Marschner (2012), such deficiencies disrupt the hormonal signaling and cellular processes essential for flower differentiation and fruit enlargement, thereby exacerbating the reproductive constraints observed under aquaponic conditions. The pH buffering capacity of aquaponic systems critically influences nutrient speciation and microbial stability. While nitrification tends to acidify the medium, fish feed inputs help maintain near-neutral pH; operation within pH 6.2–6.8 favors Fe, Mn, and Zn solubility, whereas pH ≥ 7.2 enhances nitrification efficiency but restricts micronutrient availability (Goddek and Körner, 2019; Resh, 2012). In addition, microbial biofilms formed on filter media and root surfaces contribute to nutrient mineralization and Fe mobilization, enhancing nutrient bioavailability under low ionic conditions (Bartelme et al., 2018). Therefore, multiple interacting factors appear to influence the reproductive growth of cherry tomato plants under aquaponic cultivation, making it difficult to attribute the observed growth responses to a single causal factor.
Although the aquaponic system did not produce a significant enhancement in plant growth compared with the conventional hydroponic system, its broader agronomic and environmental implications merit consideration. Beyond growth performance, aquaponics offers distinct advantages in terms of nutrient recycling and input efficiency (Nuwansi et al., 2021). The reuse of fish effluent as a nutrient source substantially reduces dependence on synthetic fertilizers, thereby decreasing production costs and minimizing nutrient discharge into the environment (Yep and Zheng, 2019). Such resource-circulating characteristics contribute to both economic feasibility and ecological sustainability (Engle, 2016; Quagrainie et al., 2018). Consequently, even when plant growth responses are comparable to those observed in conventional hydroponic cultivation, aquaponics remains a promising alternative that aligns with the principles of sustainable and cost-efficient agricultural production.
Conclusion
This study demonstrated that koi-based aquaponic systems offer early-stage physiological advantages—such as enhanced leaf expansion and chlorophyll content—but fall short in supporting later reproductive growth and fruit formation in cherry tomato plants, likely due to suboptimal concentrations of essential mineral nutrients. These findings are consistent with those of previous studies that identify potassium, calcium, and nitrate as key limiting factors in aquaponic cultivation of fruiting crops. Importantly, this study provides empirical evidence that a koi-based aquaponic system can sustain stable early vegetative growth of cherry tomato plants under temperate greenhouse conditions, indicating its potential applicability as a sustainable cultivation method in integrated aquaculture-hydroponic systems.
In summary, koi-based aquaponic systems may improve early-stage physiological performance but fail to fully meet the high nutritional demands required for sustained fruit development. To enhance system productivity, targeted nutrient supplementation (e.g., K2SO4, Ca(NO3)2) and decoupled loop configurations that allow independent nutrient optimization for plants—without compromising fish welfare—are recommended. Future studies should assess cultivar-specific ion requirements, quantify microbial interactions in the rhizosphere, and evaluate the techno-economic feasibility of modular decoupled systems under temperate, ornamental-fish-based conditions to ensure the long-term stability and commercial viability of aquaponic cherry tomato production.
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