Establishment and evaluation of an indoor aquaponics system for culturing giant mottled eel (Anguilla marmorata) with apple mint (Mentha suaveolens)

Aquaponics integrates fish and plant cultivation in a closed-loop system in which nitrifying bacteria convert fish waste into plant-available nutrients, conserving water and reducing environmental impact. This study evaluated the effects of indoor aquaponic effluent, with and without nitrifying bacteria, on the growth of apple mint ( Mentha suaveolens ) and giant mottled eels ( Anguilla marmorata ), including eel growth performance, blood biochemistry, intestinal histology, proximate composition, and microbial community structure in eel intestines and water, using metagenomics. Four experimental groups (T 0 : water-apple mint; T 1 : water-eel; T 2 : eel-apple mint; and T 3 : eel-apple mint-bacteria: Nitrosomonas sp. and Nitrobacter sp.) were conducted in triplicate for 8 weeks. T 3 yielded the highest eel weight (330.43 ± 9.65 g), surpassing T 2 and T 1 , while groups demonstrated 100% survival. Growth performance parameters, hepatosomatic index, and eel proximate composition (moisture, crude protein, crude lipid, and ash) differed significantly among groups (P < 0.05), with T 3 showing the most favorable outcomes. These improvements in T 3 were accompanied by superior water quality, bacterial abundance, and intestinal histomorphology. Apple mint growth varied significantly, with T 3 yielding the highest (P ≤ 0.01; 567.92 ± 21.31 g m −2 ). Metagenomics revealed distinct microbial communities in T 3 , predominated by Firmicutes and beneficial genera such as Romboutsia and Cetobacterium , and principal component analysis confirmed clear clustering by treatment. These findings support the successful establishment of the first indoor aquaponics system integrating giant mottled eel, apple mint, and nitrifying bacteria, highlighting the potential of nitrifying bacteria to enhance water quality, modulate microbiota, improve eel growth, gut health, and proximate composition, and increase plant productivity.