Aquaponics, the integration of aquaculture and hydroponics, can combine a wide range of suitable fish and plants. The composition of the fish feed and the respective fish physiology are the main factors affecting nutrient availability inside the system. So far, species of Perciformes in particular “Tilapia” are the mostly frequently used fish in aquaponics (Love et al., 2015, Rakocy et al., 2006). Blue tilapia (Sarotherodon aurea) was used by Watten & Busch (1984), hybrid tilapia (Oreochromis mossambicus × Oreochromis niloticus) by McMurtry et al. (1997), Nile tilapia ‛red strain’ hybrid (Oreochromis niloticus × O. aureus) by Kotzen & Appelbaum (2010) or the red tilapia (Oreochromis sp.) by Liang & Chien (2013). Most often, Nile tilapia (Oreochromis niloticus) was used, e.g. by Rakocy (1989a), Seawright et al. (1998), Essa et al. (2008), Graber & Junge (2009), Rennert et al. (2011), Simeonidou et al. (2012) and Palm et al. (2014a). In contrast, members of Siluriformes such as African catfish (Clarias gariepinus) were newly investigated by Endut et al. (2009) and Palm et al. (2014b) and hybrid catfish (Clarias macrocephalus × C. gariepinus) by Sikawa & Yakupitiyage (2010). These studies demonstrated the outstanding importance of Perciformes and Siluriformes for aquaponical production systems.

Culinary herbs are the best choice for aquaponical plant production with the highest level of income per unit of culture area per unit time (Rakocy, 2012). According to Love et al. (2015), basil (Ocimum basilicum) was the most frequently used crop under commercial aquaponic production. Basil production was studied in an outdoor commercial-scale raft aquaponic system in the Caribbean (University of the Virgin Islands, USA) with a mean yield of 2.0 kg m^− 2 in batch production (Rakocy et al., 2003) or an annual projected basil yield of 25.0 kg m^− 3 (Rakocy et al., 2004). Kotzen & Appelbaum (2010) assessed qualitative and quantitative growth parameters of basil and other herbs in brackish and freshwater aquaponic systems under desert conditions of the Negev. Not all plant species showed good growth, which indicated benefits of selective plant species cultivation. Only moderate growth was observed in basil (Ocimum basilicum) and purple basil (Ocimum basilicum ‘purparescens’) in contrast to better growth of parsley (Petroselinum sp.) in freshwater and partly in brackish water. Campos Pulido (2013) found reduced growth of Petroselinum crispum with higher salinities and effluents from Oreochromis niloticus × O. aureus production. Danaher (2013) showed no differences of plant growth parameters in parsley (Petroselinum crispum) between an inorganic fertilisation and a leachate from a geotextile bag dewatering with aquaculture effluents from Nile tilapia (Oreochromis niloticus). Buzby et al. (2016) reported no different growth of P. crispum with the use of effluents from rainbow trout (Oncorhynchus mykiss) under a low, high and amended high flow treatment. Marjoram (Origanum majorana) was rarely investigated under aquaponical production. Hundley (2013) reported favourable growth of O. majorana in combination with O. niloticus in Brasilia. Thus, O. basilicum has a wider importance for aquaponical production, while P. crispum and O. majorana are newly introduced in integrated production systems.

In coupled aquaponics fish species selection and the choice of cultivated plants may influence system stability parameters. However, investigations on effects of different fish species or Crustacea onto the same plants are rare. Sace & Fitzsimmons (2013) found better growth of vegetables with freshwater prawn (Macrobrachium rosenbergii) and Nile tilapia (O. niloticus) reared together in polyculture in a coupled raft aquaponic system. With prawn the highest yield was observed in pac choi (Brassica rapa) followed by Chinese cabbage (Brassica rapa pekinensis) and lettuce (Lactuca sativa). Water quality parameters showed better values for plant growth in combination with M. rosenbergii with a higher aquaponic system stability. A most recent study with different fish species (C. gariepinus, O. niloticus) and basil (Ocimum basilicum), lettuce (Lactuca sativa), cucumber (Cucumis sativus) and tomato (Solanum lycopersicum) by Palm et al. (2014b) demonstrated better growth of plants with significant greater yield of lettuce and cucumber fruits in combination with O. niloticus. The yield of basil showed a better mean output of 159.00 g (± 96.91) with the Nile tilapia in contrast to 117.94 g (± 92.30) with the African catfish. These studies demonstrated the influence of fish species and Crustacea choice on plant growth and system stability parameters. For the understanding of aquaponics biotechnology and the diversification of combined fish and plant production further studies are necessary.

The aquaponic set-up used in the present study was adopted from Palm et al. (2014a) with two identical coupled small-scale aquaponic units with an 8 m² gravel substrate hydroponics and a total water volume of 3.75 m³. Dissolved oxygen was identified as the most important water parameter which was used as an indicator of system stability due to lower oxygen levels as a result of higher nutrient loads during the production period. Palm et al. (2014a) subdivided their experiment of 160 days into three sub-experiments (SE I: 49 days: “run in phase”, SE II: 56 days: “exponential phase” and SE III: 55 days: “steady phase”), depending on the oxygen curves under cultivation of small sized O. niloticus and different plants like tomato, basil or aubergine. During their experiment, the feed input constantly increased (run-in and exponential phase) until the oxygen level dropped significantly, demonstrating a system nutrient overload. The reduced feed input level of 200 g per day resulted in a balanced production with a relatively stable oxygen level (steady phase). The aquaponics from the present experiment differed from Palm et al. (2014a) in simple system changes, with the exclusion of a 60 L biofilter, the increase of 20% hydroponic surface (from 8 to 10 m², addition of one plant box) with a total water volume of 3.81 m³ and a better irrigation system for the nutrient enriched water into the hydroponics.

The main purpose of the present study was a comparison of an aquaponic cultivation of Nile tilapia red strain (Oreochromis niloticus) and African catfish (Clarias gariepinus) on plant growth. Two identical low-tech warmwater, coupled ebb-and-flood aquaponic units with the use of gravel substrate adopted from Palm et al. (2014a,b) were tested in a medium temperate glasshouse during wintertime conditions in the northern hemisphere. Cultivated plant species were basil (Ocimum basilicum), parsley (Petroselinum crispum) and marjoram (Origanum majorana). The experimental design included an increasing feed input to identify the changing water parameters inside the sampled system. The effects of the omission of the biofilter, extension of the plant units (20%) and addition of a drip-water overhead distribution onto (I) the maximum possible daily feed input, (II) water parameters, (III) plant growth, and (IV) system productivity are discussed.