Bought it at "The Pond Guy", they've got a website and a catalog. Best deal I've found.

I can't give you a scientific explanation Gundude, but a couple years ago my 1 acre pond was filling up with weeds growing from the bottom. Copper sulfate and such didn't work, talked to some friends, bought $600 worth (3-8oz containers) of Sonar and it cleaned them out good.
Just picked up a new place with a small pond that has heavy weed growth, so I'm gonna nuke it pretty soon, before they get started strong.

Good stuff ! Just dump it in the pond anywhere and it will clean the whole pond in time. No adverse effect on the fish either.

For floating algae I use Cutrine (spelling?) based on Roger's advice from a couple years ago and that stuff does the trick pretty good, spray applied right on the algae mats.