The study was conducted on Długie Lake (area 26.8 ha, maximum depth 17.3 m), located in the town of Olsztyn, in north-eastern Poland (the Masurian Lake District). For 20 years starting in the 1950s, Długie Lake was used as a receiver of raw domestic and storm sewage in quantities oscillating between 350 and 400 m3 day–1. This led to complete degradation of the lake, known as saprotrophy. After some preliminary protective treatments in the catchment, the lake has been renewed by artificial aeration with thermal destratification and the phosphorus inactivation methods. Long-term reclamation of the reservoir has resulted in distinct and durable improvement of water quality. Before the restoration, the average phosphorus concentration in the surface water layer was 0.079 mg P L−1 and in the over-bottom water it reached 2.277 mg P L−1. The total phosphorus (TP) level also was very high, i.e. up to 3.5 mg P L−1. After the restoration, these values have declined to 0.001–0.017 mg P L−1 in the case of mineral P, and the current TP concentrations do not exceed 0.350 mg P L−1.