The reuse of effluent waters and sediments from African catfish (Clarias gariepinus) recirculation aquaculture systems requires a deeper understanding of the nutrient and energy flows and material pathways. Three semi-commercial systems, differing in stocking density, were sampled for nutritive and pollutant elements of the input- (tap water, feed) and output pathways (fillet, carcass, process water, sediments) by ICP-OES/MS and calorimetry. Highly water-soluble elements, e.g., potassium, accumulated in the water, whereas iron, copper, chromium and uranium where found in the solids. Feed derived phosphorous was accounted for, 58.3–64.2% inside the fish, 9.7–19.3% in sediments, and small amounts 9.6–15.5% in the process waters. A total of 7.1–9.9% of the feed accumulated as dry matter in the sediments, comprising 5.5–8.7% total organic carbon and 3.7–5.2% nitrogen. A total of 44.5–47.1% of the feed energy was found in the fish and 5.7–7.7% in the sediments. For reuse of water and nutrients in hydroponics, the macro-nutrients potassium, nitrate, phosphorus and the micro-nutrient iron were deficient when compared with generalized recommendations for plant nutrition. Low energy contents and C/N-ratio restrict the solely use of African catfish solids for biogas production or vermiculture. Using the outputs both for biogas supplement and general fertilizer in aquaponics farming (s.l.) (combined with additional nutrients) appears possible.

In mammalian research, the implementation of the 3Rs is ubiquitous. However, the adaptation of procedures for experimental work on fish seems less extensive in comparison, even though fish model organisms are common in a broad range of research fields already. To strengthen animal welfare in experimental research, we provide an overview of current research results, including studies on the nociception of fishes. Furthermore, we describe the potential of implementing the 3Rs in fish experimental research. In the context of "Reduction", we show alternative research methods to lethal sampling. Considering "Refinement", we point out possibilities to improve fish handling and indicate that adaptations to the individual species ecology are necessary. Under the aspect of "Replacement", we describe the high potential of cell cultures that can be obtained from fish tissue and give an overview of the already extensive use in ecotoxicology and virology. In addition, we illustrate that cell cultures could also be increasingly used for basic research. Graphical abstract

Optimal crop production in aquaponics is influenced by water pH and potassium concentrations. The addition of potassium hydroxide (KOH) into the recirculating aquaculture system (RAS) may benefit aquaponics by increasing the water pH for better biofilter activity and supplementing K for better plant growth and quality. We investigated the growth, feed conversion, body composition and welfare indicators of juvenile African catfish (Clarias gariepinus) treated with four concentrations of K (K0 = 2, K200 = 218, K400 = 418, and K600 = 671 mg L⁻¹). While growth, feed conversion and final body composition were unaffected, the feeding time and individual resting significantly increased with increasing K⁺. The swimming activity and agonistic behavior were reduced significantly under increased concentrations of K⁺. Leftover feed and the highest number of skin lesions were observed under K600. We suggest that K⁺ concentrations between 200 and 400 mg L⁻¹ can improve the welfare status of juvenile African catfish. This enables the application of KOH in RAS to supply alkalinity to achieve optimum nitrification at minimum water exchange and improve the nutritional profile of the process water with benefits for the welfare status of African catfish and aquaponics plant production and quality.

Recycling of phosphorus (P) from feed input in aquaculture systems gains increasing importance, especially relating to sustainable agriculture and food production. In order to find possible areas of application of African catfish solid waste, the purpose of this study was to characterize the elemental and organic matter composition and P speciation in the aquaculture fish waste. Pyrolysis-field ionization mass spectrometry (Py-FIMS) was used to investigate the composition of organic matter and P K-edge X-ray absorption near edge structure (XANES) spectroscopy to describe the occurring P-containing compounds in African catfish solid waste from an intensive recirculation aquaculture system (RAS). The solid fish waste was mainly composed of sterols, free fatty acids and alkylaromatics, as it is common for digestive systems of animals. Ingredients such as the phytosterol beta-sitosterin confirm plant-based feed ingredients and some recalcitrance against digestion in the African catfish gut. The P in the solid fish waste was exclusively bound as calcium-phosphates. These calcium-phosphate minerals as major constituents of African catfish waste may have beneficial effects when applied to soils, suggesting the use of this waste as possible soil amendment in the future.

Ortho-phosphate inside recirculation aquaculture systems is limited as a consequence of precipitation and regular water exchange rates. To improve plant growth in coupled aquaponics, phosphate fertilizer addition to hydroponics can increase PO₄³⁻-P concentrations inside the process water. We investigated the effects of four PO₄³⁻-P concentrations (<10 (P0), 40, 80, 120 mg L⁻¹) in rearing water on growth performance, feed efficiency, and welfare traits of juvenile African catfish (Clarias gariepinus Burchell, 1822). By trend, optimum specific growth rate of 2.66% d⁻¹ and feed conversion ratio of 0.71 were observed at 40 and 80 mg L⁻¹ PO₄³⁻-P. Higher PO₄³⁻-P significantly affected skin coloration, swimming activity and external injuries, with the palest and inactive fish combined with most external injuries in the P120 group. Mineral and protein contents in the fish remained unaffected, while fat content inside the fillets enriched with increasing PO₄³⁻-P. Inorganic P in blood plasma increased significantly, while phosphate concentrations inside the fillet remained unchanged. We suggest that PO₄³⁻-P concentrations of 40 to 80 mg L⁻¹ do not reduce the performance of African catfish aquaculture, while increased values of 120 mg L⁻¹ affect fish welfare. This allows limited addition of PO₄³⁻-P fertilizer in coupled aquaponics with African catfish to support plant growth.

Heterotrophic denitrification is increasingly used in land-based recirculating aquaculture systems (RASs) to remove nitrate and minimise new-water demand, but the carbon sources traditionally used—methanol and acetic acid—raise safety, corrosion, and cost concerns. This study evaluated glycerol as an alternative external carbon source in a commercial land-based Atlantic salmon RAS in Norway. Trials in two identical post-smolt systems with integrated denitrification bioreactors compared glycerol (80%) against acetic acid for denitrification performance, nitrite accumulation, pH stability, carbon-source consumption, cost, and hazard profile. Glycerol achieved similar or higher apparent total dissolved nitrogen (TDN) removal than acetic acid and, once the biofilm had acclimated, was completely consumed. It maintained a stable outlet pH (≥6.5), avoiding the enzyme inhibition seen when acetic acid depressed pH, and proved about 1.5 times more cost-effective (≈NOK 66 vs. 97 per kg TDN removed). A standardised hazard assessment ranked glycerol as the safest option for flammability, toxicity, and corrosiveness. TDN removal correlated strongly with dissolved nitrogen gas saturation at the bioreactor outlet (r² = 0.79–0.82), indicating that total gas pressure monitoring is a promising, low-cost proxy for performance. Nitrite accumulation per unit of nitrogen removed was, however, about three times higher under glycerol (≈1 mg NO₂-N per mg TDN) than under acetic acid (≈0.3 mg NO₂-N per mg TDN), and the nine-day glycerol phase was too short to establish whether this resolves or persists as the biofilm matures. Subject to routine nitrite monitoring in the fish tanks, glycerol is a cost-effective, low-hazard, and pH-stable alternative to acetic acid for nitrate removal in salmon RASs.