Pond algae is probably one of things that most easily frustrate pond owners. The only thing more frustrating is actually controlling algae. Algae is easily one of, if not the biggest concerns for owners of new ponds.
The problem tends to occur in new ponds because they have not yet achieved the proper balance between plants and animals. New ponds are likely to have an algae bloom. The algae consume the excess nutrients in the water. You need to give your pond some time to balance itself before adding fish because they only add to the overload of nutrients.
More importantly, one of the things you have to do when setting up your new pond is to install the right equipment. The pump you use should move at least half of the total pond volume for a water garden. A water garden is a pond with many plants and a few or no fish. A koi pond has less plants and large fish so it requires more filtration than a water garden does.
It is imperative that you filter the water. Please check "Google" or your favorite search engine for the phrase, "How to Select a Pond Filter" to get detailed information regarding the criteria for selecting a filter. You should find all kinds of choices for a few different pond sizes and filter combinations. Biological filtration will take several weeks or even months before it matures to the point of improving the quality of your pond water. In order to work the filter will need to run 24 hours a day, seven days a week.
Your pond's construction is also crucial. 40% of its surface area should be used for the deep zone. The deep zone should contain at minimum 2 feet of water in a water garden and 3 feet or more in a koi pond. Your intermediate zone should be 30% of the surface and the depth should be around 1-1/2' to 2'. The remaining 30% should have a depth of 1 to1/2' feet.
To make it easier to remove debris from your pond, a slight slope to the deepest level is recommended. A pond skimmer can remove as much as 85% of the debris before it sinks to the bottom. And remember to construct your pond so that rainwater doesn't flow over your yard and into the pond. This is a common source of pond algae because rain runoff carries lots of organic debris that contains nutrients that feed algae. Chemicals from fertilizer can also cause problems for your pond.
Even if your pond is already in place, you can change the area around it to control runoff. A buildup of debris in the bottom of your pond is normal and as long as it's no more than ¼ inch you won't need to remove it. A skimmer net or algae net works well for removing string algae and dead leaves. You can also use a pond vacuum to remove debris that is too fine to be removed with the net.
If you have a water garden and not a koi pond then you will need to have the right kind and number of plants. The use of Anacharis or other underwater plants, as well as floating plants, can help to remove excess nutrients from your pond by absorbing them and using them for their own growth. By covering 2/3 of the surface with water lilies, floaters and other shade plants you will keep the water cooler by reducing the amount of sunlight that hits the pond. This in turn starves the algae for sunlight.
You can still have algae after taking all these precautions, especially when your pond is new. You may need to use other methods of controlling algae during your pond's first few years. As the pond matures the algae lessens and hopefully it will no longer be a problem.
There are some things you can do while you're waiting for your pond to mature. There are bacteria and enzyme products on the market that can improve the overall quality of your pond water by reducing sludge, thus reducing algae. You can use products that add beneficial bacteria and enzymes to your pond. If used on a regular basis, these products will not only improve your pond's overall quality, the products will also reduce odor, reduce the amount of sludge and keep your fish healthy.
If you have green water, the installation of an ultraviolet sterilizer will help to ensure that your water remains clear 100% of the time. There are some algae that can be of benefit to your pond. Filamentous algae have many algae cells connected. It can be long and stringy or short and furry or be shaped into webs or mats. The short velvet type of algae that clings to rocks and sides of your pond is useful. It gives your pond a natural look. It also makes use of nutrients from the water which provides oxygen during the day. This is the type of algae that your fish will eat.
These algae cannot be totally removed when you've got both fish and plants in your pond. String algae are a little more difficult to control. You can physically remove it from the pond wherever possible. Filamentous type algae thrives on waterfalls and in shallow streams because it gets more intense sunlight providing it with more heat and light so there is a constant supply of nutrients that flows through it. Bear in mind that fish and the food they eat provide nutrients for your pond, which in turn provide a food source for algae. Maintain a balance between your filter and the number of fish you keep in your pond and of course don't feed your fish too much food.