Michi and Jeff are right. The best way to deal with phosphates in the water is
to get your plants growing well and fast. To start out, use stem plants like
Hygro, wysteria, valisneria. Get some "anchor" plants going too, such as an
amazon sword or two. They will soon get large and suck up nutrients quickly as
well.

As Jeff and Michi pointed out, you need to make all nutrients higher plants use
available to them, so that they will out compete the algae. That means getting
good lighting, CO2, good substrate (probably flourite), and a good trace mix
like Tropica Master Grow. If you have not already, take a look at
http://www.csd.net/~cgadd/aqua/. It seems to me that his ideas are all the more
relavant when you have high phosphates..

Fact is that fish food has a lot of phosphorous in it. Even if your water
supply did not have extra, you would still need to get those plants going fast
to deal with the inevitable extra phosphorous from food.

-Bruce