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Common practice of aquaculture in Vietnam and other countries in South East Asia involves frequent discharge of polluted water into rivers which results in eutrophication and degradation of receiving water bodies. There is therefore a need to develop improved aquaculture systems which have a more efficient use of water and less environmental impact. The aim of this study was to assess the suitability of using constructed wetlands (CWs) for the treatment of fishpond water in a recirculating aquaculture system in the Mekong Delta of Vietnam. Water from a fishpond stocked with Nile tilapia (Oreochromis niloticus) and common carp (Cyprinus carpio) was recirculated through horizontal and vertical flow CWs ) at acceptable concentrations for growth of the fish. There was a good removal of organic matter measured as oxygen demand with up to 50% removal of BOD and COD in both types of CWs despite the high loading rates and low concentration levels. However, the vertical flow CWs performed better than the horizontal flow CWs as they had higher nitrification rates and higher DO concentrations in the outlets. The ornamental Canna × generalis planted in the CWs grew faster and took up more N and P in the vertical flow CWs. The aquaculture fish had a feed conversion ratio of 1.53 based on feed dry weight, and 31% and 34% of N and P input, respectively, were incorporated into fish biomass. Only minor quantities of phytoplankton algae were removed in the CWs but abundance of toxic algae such as Microcystis was low. It is concluded that particularly vertical flow CWs have great potential for treatment of fishpond water in recirculating aquaculture systems in the tropics as the discharge of polluted water and the associated environmental impact can be significantly reduced.

The feasibility of using planted biofilters for purification of recirculated aquaculture water in the Mekong Delta of Vietnam was assessed. The plant trenches were able to clean tilapia aquaculture water and to maintain good water quality in the fish tanks without renewal of the water. NH 4 -N was removed efficiently in the plant trenches, particularly in the trenches with Canna glauca L., probably because of plant uptake and nitrificationdenitrification. Plant uptake constituted 6% of N and 7% of P in the input feed. Approximately 1.0 m 3 of water was needed per kg of fish produced, and 370, 97 and 2842 g fresh aboveground biomass of Ipomoea aquatica Forssk., Lactuca sativa L. and C. glauca, respectively, were produced. The leafy vegetables provide some extra income besides fish products, whereas C. glauca provides nice flowers and contributes to a significant nutrient removal with annual uptake rates of 725 kg N and 234 kg P ha À1 year À1 . This research demonstrates that integrated recirculating aquaculture-hydroponics (aquaponics) systems provide significant water savings and nutrient recycling as compared with traditional fish ponds.

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