For the sake of continuity this section is still organized into natural and herbicidal. If you think about it logically it's pretty easy to understand this way. String algae generally grows near the surface or on waterfalls so blocking sunlight is not really an option. It is anchored so it doesn't go through the filter so UV clarifiers are also useless. The other methods for green water do work on string algae as well. Keeping nutrient levels low will slow it down and you can kill it with algaecides the same as green water algae. There are a few other products that are specifically geared towards string algae.
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.
The other methods for dealing with string algae revolve around poisoning it directly. There are a few things that are specifically designed to kill string algae that won't harm other plants.
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. The problem is this chemical breaks down within minutes of sunlight exposure. Because the straw is constantly breaking down it keeps a low level of this in at all times. 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.
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.
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 but slower than the algae off concentrate. 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.
SAB or String Algae Buster is a synthetic bacteria that has been specifically designed to kill string algae. Like most bacteria products, it is only really effective in warm weather and it takes time to work. It is, however, more thorough than most other methods. It will take care of string algae in the body of the pond as well as on the waterfall.
Algaecides are fast and effective at killing algae but they do have drawbacks. Any chemical that kills algae will also harm other plants. Algaefix by PondCare and Algae Control by Tetra are two products with the same active ingredient. They work on string algae as well as on green water. If you don't have any plants in the pond then Pond Blox by Tetra is more effective, but will kill any plants in the pond.