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Horizontal subsurface flow constructed wetlands (HSSCW) allows organic matter and nitrogen removal of fish farm effluent prior to streams discharge. The effect of macrophyte species on HSSCW efficiency was tested in ten units in a greenhouse experiment, in summer and winter. Eight units were individually planted with Phragmites australis, Typha angustifolia, Phalaris arundinacea and Calamagrostis canadensis (two units per species) and the remaining two units were left unplanted. The units were fed with a reconstituted effluent made from trout farm sludge. The sludge was diluted to obtain an average of 15 g COD/m2/d, 3 g BOD5/m2/d, 6 g TSS/m2/d, 0.50 g N/m2/d, 0.15 gP/m2/d and a resulting hydraulic loading of 3 cm/m2/d. Water quality was analysed in summer 2002 and winter 2003 for COD, BOD, TSS, TNK, NH4+, NO2- + NO3-, TP and o-PO4. Planted units were at least 5% more efficient in pollutant removal than unplanted units in summer and at least 10% in winter. The increase in removal efficiency for planted units was small, mainly because of the low loading conditions of the fish farm effluent. TKN (96% in summer and 88% in winter) and COD (96% in summer) removal were more efficient for Phragmites and Typha, the two species with large rhizomes. Phalaris was more efficient than the others with COD and BOD removal at 95% in winter. Calamagrostis was the least efficient species, with the largest difference being for winter nutrient removal. It was also the species with the lowest belowground: aboveground biomass ratio, around 0.25 compared to above 2 for Typha. Units planted with macrophytes with large belowground biomass showed less seasonal variability.