"Well, just buy some more (koi)" was his answer.

ak!
no!
Nutrients for algae are sun, new water, fish WASTE, fertilized run off, rotting
plants, blown in dirt.

You've got a lot of nutrients in the pond and they are decaying.
Is there run off coming into the pond, is it coming from an area that is
fertilized?
I'd get the water moving.
These folks are a lot of help
http://www.aquaticecosystems.com

Also
The following are great resources
for large, natural ponds.
Earth Pond Sourcebook
by Tim Matson
and the newsgroup
misc.rural
and the ask the Pond Boss forum
http://www.pondboss.com/cgibin/ubbcgi/ultimatebb.cgi

and
call your county extension agent, they can be
a lot of help.

good luck!
And let us know what happens,

k30a
and the watergardening labradors
http://www.geocities.com/watergarden...dors/home.html