I'm not familiar with ultrasonic treatments for algae, but my limited research after reading your post leads me to believe that the SS400 may not live up to your expectations. They tout it as being effective on green water algae blooms (over several days), but admit that it may take several months to break down algae "clumps", like what you have now.
Quoted from the website:
"Chara for instance, will not be harmed by a sonic wave, and other plants like duckweed are resistant as well. As we mentioned earlier, filamentous or string algae is often found in thick clumps or masses. When you have algae like this and treat it with a sonic system, be prepared to wait awhile. It's not a quick fix. Our findings showed that it may take several months before the device can work it's way through the growth and it wasn't uncommon for the pond to look a bit worse before it started to appear better."
In my opinion, waiting several months and then claiming improvement could be due to the FA's natural response to varying water temps, not so much from the introduction of ultrasonic treatments.
