Abstract Phosphorus (P) accumulation in a closed recirculating aquaculture system ({RAS)} was studied using a goldfish tank as a model. It was found that the accumulated P in this system was soluble inorganic phosphates (Pi) and the highest concentration was up to 8 mg {P/L} after 40 days of fish cultivation. Phosphorus in the water was increased linearly with the rate of 0.19 mg {P/L/day.} A mutant strain of the cyanobacterium Synechocystis sp. {PCC} 6803 ({ΔSphU)} that lacks the {SphU} regulator of the Pho regulon could decrease Pi in the wastewater of {RAS} to the concentration below the P detection limit of 0.01 mg {P/L} at the rate of 2.07 ± 0.33 mg P/h g {DW.} This was corroborated by the increase of cellular polyphosphate and P content in the {ΔSphU} strain as revealed by fluorescence microscopy. After the first cycle of P removal, the cyanobacterial cells were recovered from wastewater by cell flocculation using chitosan. The flocculated cells could be reused for efficient P removal for the next 3 cycles.