Abstract.The feasibility of using a smalt nutrient‐film hydroponic technique hydroponics unit as a mineral filter for a recirculating aquarium system has been Investigated. The aquarium in question employed microbial biofilters to convert ammonia and waste materials to a mineralized form, particularly nitrates.A recirculating NFT system was designed using a 40‐l capacity catchment refilled at weekly intervals from the aquarium, which was stocked with common carp,Cyprinus carpioL. and rainbow trout,Oncorhynchus mykiss(Walbaum). Four crop species were grown (garden peas,Pisum sativumL., lettuce,Lactuca sativaL., watercress.Nasturtium officinaleR., and barley,Hordeum vulgareL.) with no additional nutrient supply. A control system was set up using tap water. Although yields were not as high as seen in commercial NFT systems the aquarium water‐fed system produced viable plants, with higher yields and better quality than seen in the controls. At the same time mineral levels, especially nitrates, were reduced.If the unit was incorporated into the aquarium filter system as a single passage unit, water quality could be maintained, together with the production of a useful crop. The data obtained in the present study are compared with previous reports on the use of hydroponics in conjunction with fish cu