I've used it for the last 3 years and the only string algae I get is in the waterfall... I bought a bale of it at the feed and grain store, so I make large sackfuls of it in nylon mesh bags and put them in my two Skippy-type garbage can filters... I see the small little bags they sell at the pond stores, and I didn't think that was enough for my sized pond (14X32)... The bale I bought was 12 bucks and I probably have enough for 3 years...
Wouldn't testify in court that it works, but I'm not gonna stop using it, because I used to have alot of string, now none at all... Could be my pond has balanced out, or my plantings are taking care of the water... But, just saying.......
Dave