This page explains briefly what to do about algae growing in aquariums, fish bowls, and ponds. More importantly there is also information about why algae is growing in your fish's home.
 
"Your fish's home" might be an aquarium, a fish bowl, or a pond. The comments on this page apply to all three types of fish homes.

1. A Short Answer
If the water in your fish's home is green, then algae is growing in the water, which shows that the water contains too much dissolved fish waste. You should change some of the water in your fish's home every day, until the water in your fish's home is clear again.

How much water should you change? Don't overreact and change too much water at one time. Change 20% of the water once each day until the green is gone, then change 20% of the water twice a week.

Don't change more than 20% of the water in one day, unless you have a catastrophe, and green algae is not a catastrophe. Click here for information about changing water in your fish's home. 

Thank you Dan for your explanation. There are several types of algae, that grow in fish's homes. Some types of algae float and multiply in the water. Other types of algae grow on objects such as the rocks, ornaments, gravel, plants, and the sheets of glass or plastic in your fish's home.

All Algae is not Green. There is also black algae, brown algae, red algae, and several other color variations. Everything written on the this page about green algae applies to the other color varieties of algae too.

3. Why is Algae Growing in My Fish's Home?
Algae needs water, light, and nutrients. If your fish's home has too much algae, we know your water is getting lots of light and has lots of nutrients dissolved in it. Where did the nutrients come from? Fish eat food and produce waste that is an excellent nutrient for algae.

4. How Do I Get Rid of the Algae?
There are four approaches to getting rid of algae. Get some fish to eat it, poison the algae with chemicals, reduce the amount of light, or starve the algae by reducing the amount of nutrients in the water.

Algae Eating Fish such as Plecostomus Catfish and Siamese Algae Eaters are both good aquarium fishes that will eat some types of algae. But they will usually not eat all the algae in an aquarium, and we don't have a fish available that will filter out the algae that grows in water and makes the water green. Most aquarists enjoy having one or both of these types of fish, but an algae eating fish usually won't solve an algae problem.

Plecostomus Catfish do not eat all types of algae, and you may have a type of algae that your Plecostomus does not like to eat. Do not depend on a Plecostomus Catfish or any other species of fish to solve your algae problem.

A better method is to reduce the amount of light and the amount of nutrients in the water.

Reduce the Amount of Light.
If you have a fish bowl, move it away from the source of light such as a sunny window.

If you have an aquarium, keep the light turned off as much as possible. If your aquarium is near a sunny window, move the aquarium. If the aquarium is difficult to move, cover the side of the aquarium that faces the sunny window with aluminum foil (sold in grocery stores for wrapping food).

If you have a pond, you may need to cover it with lath or a special netting several feet above the pond to block the sunlight. Both the lath and the netting for this purpose are sold at home improvement centers.

Putting a pond lily or other floating plants such as Water Hyacinths in your pond will also reduce the amount of light that gets to the water. But do not try to reduce the amount of light hitting your pond by putting a plastic cover directly on the water, because the pond needs to be exposed to air or the fish will soon use up all the oxygen in the water and may suffocate.

Reduce the Amount of Nutrients
in the water by changing some water every day. The most effective way to get rid of algae is to reduce the amount of nutrients in your water.

Algae must have nutrients to grow. Your fish have been eating food and producing fish waste that dissolves in the water to become nutrients for the algae. Start changing water in your fish's home every day until the water clears and the green algae is gone. By changing the water, you'll remove the nutrients from the water and starve the algae. 

Don't get impatient and change too much water at one time, because that can shock and harm your fish. Change at most 20% of the water each day until the green is gone, then change 20% twice a week. Don't change more than 20% of the water in one day unless you have a catastrophe. Green algae is not a catastrophe. Click here for more advice about changing water.

Slowly over a period of days the algae will disappear. You can put the water from your fish's home on the plants in your yard, so you won't be wasting water. When the water in your fish's home clears and most of the algae is gone, you can go back to changing water twice a week instead of every day.