The present experiment was designed to simulate an on-going field trial at Au Tau Substation, Agriculture and Fisheries Department, Hong Kong Government, using a series of fishponds to treat polluted riverwater, as well as for promoting algal growth, before the water was used for rearing freshwater fish. It was revealed that aeration was necessary to lower the level of ammonia. It also accelerated the conversion of ammonia to nitrite, and then nitrite to nitrate. Substantial amounts of metal (K, Mg, Cu, Zn) were also removed from the riverwater, possibly by algal assimilation. The algal products contained a high crude protein content (42% dry weight basis) which could be used to feed carp and tilapia.