Barley Straw Extract for Green Water Algae ?

Does anyone have experience treating "green water algae" infestations with
barley straw extract ? What were your conclusions ? Did it work or not
work ?

Also, does it actually *kill* the algae ? Or does it just clarify the water
in some other way ? I have a 55gal tank and the bottle I was looking at
online at www.thatfishplace.com says it treats 2,500gals. So that's making
me think now that I have the infestation, I'll have to treat it forever and
ever.

If barley straw extract ISN'T the best way to deal with green water algae,
what is ?

Thanks - Shawn