Forgive me for chiming in here, but I've worked with ultrasound for several years now. It won't work on Chara or aquatic weeds at all. There are a few specific species of algae and this includes both FA and single cell that ultrasound won't control because of their cell structures.
Also it normally just damages the type that will cause green water, as it ruptures their air bladder. This is fine if the pond has some depth to it, but shallow water still provides ample access to sunlight and that will keep the algae alive quite a bit longer.
From everything I've seen, both in application and practice, it's safe for fish. The frequency of the sound waves it creates are pretty well tuned to algae resonance. I can say that when it fits the need, it can work exceptionally well on larger waters...meaning bigger than a backyard koi pond. It's being used more and more in industrial settings as well such as waste water, power plant cooling intakes, etc.
But it's not a silver bullet. We actually started a rental program last year for these because despite my request for water and algae samples and looking at these under a microscope to try and qualify a particular setting, nothing will actually confirm results like putting it in the actual pond.
I hope this helps clarify things a little bit.
Mark
