Balancing Naturally: Reducing available fertilizer.
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.
When trying to balance out a pond it's important to understand the relationship between plants and fish. Fish food is the biggest source of fertilizer in a pond. Sometimes the best solution is to reduce the number of fish.
Method 1: Using Plants to Consume Excess Fertilizer
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.
Physically Killing String Algae: Using a chemical to poison algae
The other methods for dealing with string algae revolve around poisoning it directly. Some of these are gentle and won't hurt other plants. Others will do alot of damage. Every pond is different and no one knows your pond better than you. Over time you'll figure out what works best for your water garden.
Method 2: Barley Bales
Barley straw has been used for centuries to control algae in ponds. As the barley straw rots it releases a chemical that inhibits the growth of new algae cells. It takes time for barley bales to start working, but once they get going one will keep working for about 6 months. Putting one in twice a year is usually sufficient to keep it under control.
Barley extracts are exactly what they sound like, a liquid extract of the chemical produced by barley straw. It is quicker than putting in a barley bale. It's a weekly dose that will control it but needs to be reapplied most of the summer so it can get expensive after awhile.
Method 3: Chemical Oxidizers (quick fix)
Algae Off is a chemical version of a barley bale. It is a white powder that physically burns string algae off of the rocks. It needs exposure to air to be effective so it only works well on waterfalls, but it will burn off the algae in a matter of minutes. It also doesn't have any lasting or harmful effect so it's basically a quick fix.
Method 4: Chemical AlgaecidesAlgaecides are fast and effective at killing algae but they do have drawbacks. Any chemical that kills algae will also harm other plants. Algaefix and Ponds Plus brand Green Halt are two products that work great. They work on all types of algae, but they can be hard on delicate plants. If you don't have any plants in the pond then Pond Blox are more effective.