Abstract: This invention offers an effective method of inhibiting the growth of toxic bloom algae caused by cyanobacteria Mycrocystis aeruginosa in freshwater environments comprising administering an effective amount of 5,6-dichlorogramine to the freshwater. Also, the invention offers an effective method of inhibiting the growth of toxic bloom algae caused by M. aeruginosa comprising administering an effective amount of 5,6,7-trichlorogramine to the freshwater. Both compounds, 5,6-dichlorogramine and 5,6,7-trichlorogramine, may be administered individually or in combination to the freshwater. Both compounds are individually very potent and only need to be administered at a very low concentration to be effective at inhibiting the growth of M. aeruginosa in freshwater environments. Both 5,6-dichlorogramine and 5,6,7-trichlorogramine are each individually effective at inhibiting M. aeruginosa at low concentrations, between about 0.01 mg/L and about 0.05 mg/L, or more specifically at a concentration of about 0.03 mg/L.