Phosphorus is one of the major nutrients that regulates microbial growth in water systems, containing a fraction that is easily utilized for bacterial processes known as microbially available phosphorus (MAP). However, its effect on opportunistic pathogens such as Legionella bacteria is unclear. In this case study, the impact of MAP on Legionella spp. was investigated. A point-of-use (POU) ferric hydroxide sorption filter was used to reduce MAP levels in the internal drinking water supply (DWS) of a multistorey residential building in Riga, Latvia. The study included two distinct domestic hot water (DHW) heat exchanger setpoints: initially set at 57 °C for 12 weeks, and then lowered to 48 °C due to a decision to reduce energy consumption. Prior to the startup of the POU device, the internal DWS underwent centralised chemical flushing and hydrogen peroxide disinfection, which proved insufficient for long-term Legionella control. The POU device successfully removed around 70% of MAP to a concentration of 3.6 μg l−1 (SD 1.5 μg l−1), nevertheless, Legionella pneumophila serogroups 1, 2, and 3 were identified. During the initial high-temperature period, similar concentrations of L. pneumophila were detected in both buildings, regardless of the presence of the POU device. However, at the lower setpoint, Legionella concentrations increased by more than tenfold in the MAP-limited environment. This was attributed to the opportunistic pathogen's higher growth rate compared to native bacteria experiencing a nutrient deficiency. In summary, reducing phosphorus levels alone is insufficient for effective control of Legionella bacteria and additional strategies are needed to address the complexities involved in Legionella control within DWS systems.