Synthetic dyes, methylene blue and toluidine blue, are used in medicine for coloring histological preparations. Illumination of these compounds with a monochromatic purple lamp results in the excitation of these molecules and cytotoxic compounds are formed. This process is called a photodynamic effect. The aim of this study was to determine how the synthetic dyes, depending on the hardness of the water, act on the selected microorganism, Legionella pneumophila. It is a gram negative bacteria that is present in ground and surface waters. People are infected by the inhalation of contaminated aerosols, and Legionella can cause Pontifical fever, and severe systemic disease, Legionnaires disease. Legionella form biofilms in water pipes, which protects it against the effects of different external conditions. In this study, the effect of synthetic dyes was investigated on four samples of different water hardness. The efficacy of synthetic compounds was investigated by determining minimum effective concentration (MEC), photoinactivation test, inhibition of adhesion to polystyrene, inhibition of biofilm formation, and breaking the already formed biofilm. It is apparent that both synthetic compounds show an anti - Legionella effect and that the antibiophilic effect does not depend on the hardness of the water. The presence of ions is noticeably favored by the synthetic color of toluidine blue effect, while the methylene blue is vice versa. The highest bacterial reduction is present in 10 minutes of monochromatic purple light exposure. Inhibition of adhesion, inhibition of biofilm formation, and degradation of the already formed biofilm, there is a significant difference in the number of bacteria in samples of water with the tested compounds compared to the control.