The first type of algae is the small, single-celled type that gives water a green tinge. This is the most annoying type and what most products are geared to control. There are a few different ways to look at algae control, but it can be broken down into two categories; Balancing Naturally and Physically Killing.
When you look into a farm pond what do you see? Usually the water is green or brown and sometimes has a layer of green algae floating on the surface. Have you ever gone on a float trip in Southern Missouri on the Meramec or Black Rivers? The water is always crystal clear and very little algae is growing on the rocks. So what's the difference? In those fast flowing rivers there are very few nutrients to feed algae. The fertilizers from the fish waste and rotting vegetation get swept away down stream. In a regular pond the fish waste and rotting vegetation is consumed by bacteria and converted into nutrients. These nutrients can be used by any plants, but in the absence of many plants they will get used by the algae.
Plants use up the nutrients in a pond reducing available fertilizer for algae. The best plants for this are Water Hyacinths, Water Lettuce, Anacharis, and Hornwort. These are extremely fast growing plants that are able to outcompete algae. Any plants whose roots are exposed to the water (not in a pot) will help. There are a couple other factors to consider when using this method. First, the more fish there are in the pond and the more you feed them the more difficult it will be to balance your pond. Every handful of fish food is a handful of fertilizer. Second, large fish love to eat plants. If you have big Koi you will need a way to keep the plants separate from the fish. Having a bog area that is separated from the main body is a good way to accomplish this. Rotting vegetation on the bottom is also converted into nutrients. Controlling Sludge by keeping the bottom free of leaves and debris is also very important.
There are live bacteria products on the market that control algae in the same manner. They consume the nutrients to starve algae. This method involves adding liquid bacteria solutions weekly to the pond, such as Ultra Clear, Aquazyme, and Bacteria & Barley Enzyme Cleaner. Just as with plants, the more fish there are in the pond the more difficult it will be to keep clean.
Pretty simple concept. If the algae can't get light, it can't grow. This is accomplished by dying the water using Aqua Shade or Bio Black. The down side is the water is so dark you can't see your fish. So you may wonder, why would anyone want this? It is a good way to keep the water clean and help protect fish from predators during the winter months. Several 30% water changes will thin out the dye quickly when you are ready to start the pond back up in the spring.
The other methods for dealing with algae removal revolve around poisoning it directly. These methods are faster and easier than balancing the pond naturally, but they are also harsher to other plant life and generally more difficult to maintain. There are two ways to do this; putting a chemical algaecide in the pond, and/or putting a high intensity UV light on the filter.
Algaecides are fast and effective at killing algae but they do have drawbacks. Any chemical that kills algae will also harm other plants. There is one chemical on the market that doesn't do much damage to plant life. They got around the problem by making the molecule so large that it doesn't travel through dirt in a pot, preventing the algaecide from reaching the root system of plants. Any plants whose roots are exposed to the water will still be affected, but the potted lilies and bog plants will not. The name of the active ingredient is too long to put in here, but there are several brands on the market that use this same chemical. Algaefix by PondCare and Algae Control by Tetra are two we commonly recommend. If you don't have any plants in your pond then Pond Blox by Tetra is more effective, but it will kill all aquatic plant life.
The final method for controlling green water is the UV Clarifier. This is by far the most effective and fastest, but it is also the most expensive. It is a high output UV light that kills anything that runs through it. The biggest drawback is that the bulb will lose power over time and needs to be replaced once a year. One other problem is that it ONLY kills what goes through the filter; so string algae and smooth coating algae will not be affected. Since you are killing the green water algae you're also leaving the nutrients behind which can result in more string algae growth. Some ponds never experience problems with String Algae, though most do.