All good answers. I'm going to add my algae primer
so you can get a basic understanding of the beast.

Algae is normal and natural and needed by the lower
end species in your pond. One of the bulding blocks
of life. Our problem is when things get out of balance,
very easy in backyard ponds, and algae goes gonzo...

Green Water is caused by single cell free floating suspended in the
water column algae. String algae is long, flowing, likes moving water
and has some body to it. Substrate algae is like a fuzzy green sweater
and grows on rocks, liners, plant baskets, and is considered a good
algae as it keeps the suspended and string algae at bay. It also hosts
lots of tiny zoo plankton, insect larvae, worms and other tasties that
are good for fish to consume
along with their veggies (the algae).

All algae thrives on sun, fresh water, fish waste, fertilized run off,
rotting plants and blown in dirt. In new ponds and spring ponds algae
is always the first thing to start growing.

The best defense against algae is to have lots of plants to compete for
the nutrients, few fish, not overfeeding those fish, some shade,
blocking run off and cleaning up debris.

Do not use algaecides, they only make lots of suddenly dead algae to
feed the next algae bloom. Gently remove string algae. Don't worry
about fuzzy algae that grows on the sides of things.

Most algae blooms will pass within a couple of weeks.
Time and patience is key.
Remember patience...

kathy :-) www.blogfromthebog.com
this week ~ bladderwort

Pond 101 page for new pond keepers ~
http://hometown.aol.com/ka30p/myhomepage/garden.html