Fishlover asked this a time back.
"Has anybody tried Phoslock for getting rid of green algae in
fishponds?"

Well after reading the various responses in particular the one about Hay, I
drove down a dirt track in the Hawkesbury and cut some wild oat hay.
Obvoiously seeded from passing trucks. Didn't know how much but I tied 3
bundles about 9" long half as thick as my wrist. The surface area is approx
2 square metres with various depths from beach to 60cm. It was pea green,
bloody awful. Seven days have apassed and I can now see the various depths.
Magic! Still has a bit to go but I have no doubt it is working.
It has a couple of small Koi, haundreds of relocated small river fish and 4
frogs. About a thousand taddies in smal buckets about the garden; spawn
taken prior to a smorgasbord for fish.

Cheers.