Algae have been proven to be a severe health hazard to humans, aquatic and semi-aquatic animals. Chemical methods available to control the algae have unwanted side-effects. For this reason, ultrasonic algae control has been under investigation. We measured the eradication effectiveness of ultrasound at three typical centre frequencies. At all three frequencies physical damage to the algae was observed. We conclude that it is possible to eradicate blue-green algae in the clinical diagnostic range. Taking into account the geometry, the low attenuation in water, and the NATO Undersea Research Centre for Human Diver and Marine Mammal Risk Mitigation Rules and Procedures, even at these low voltages, the safe swimming distance is at least several meters away from the sound source.