Ultrasound works on algal ...

Ultrasound works on algal blooms by rupturing the vacuoles that are used for flotation and are integral to the diurnal rise and fall of oxygen/carbon dioxide cycle (Blue-Green algae--cyanobacteria). With the vacuoles ruptured the algal cells settle on the bottom of a lake where they provide a food source for bottom feeders in the pond or lake.  Contrary to frequent popular speculation they are not harmful to fish, turtles, and other biota living in the pond, as it doesn't function like sonar on ships.  Most companies that make ultrasonic systems do not release or publish their frequency spectrum as they consider them proprietary and to give out the frequency would be akin to giving money to a competitor to mimic their design.