First before everything else you must have a good filter system. I prefer the Biofall/skimmer filtration system.
The skimmer removes the larger debris from the surface of the pond and gives a clean place for the pump to sit so it does not get clogged.
The biofilter is where the water gets filtered. The water usually enters the biofilter and passes through filters and some sort of media that supplies a large surface area for bacteria to grow (lava rock/ribbon/bioballs/sponges). This is also a great place for plants and is usually where you waterfalls begin.
Plants compete with algae for food. So the more plants you have in the pond the better. Please view what are the invasive plants in your area and don’t buy or use them. I like to float water hyacinth in my biofalls because the long roots feed on the nutrients the filter traps.
Bacteria and Enzymes are also used to breakdown and use up the nutrients in with water. In the spring we add beneficial bacteria to the pond because the cold weather decreases the numbers over the winter. If you drain and power wash you pond you also kill off the good bacteria so you need to add more.
Barley straw placed under a falls or pellets put in the biofilter help deter string algae, it is a deterrent. As the barley breaks down in the presence of bacteria, sunlight and oxygen, the compounds are converted into low levels of hydrogen peroxide. The hydrogen peroxide modifies the pond chemistry to improve pond clarity. Hydrogen peroxide itself is relatively unstable and does not persist in water, but the slow release effect from the barley straw provides a constant source of hydrogen peroxide. The effects from barley straw will begin as early as one to two weeks after installation, and will remain effective for between three to four months.
Green Clean is a product with organic approval that is a granular based peroxide. It works on contact with algae. I use it on waterfalls and when spring cleaning a pond. READ THE LABEL BEFORE USE!!!!
I have some clients with large koi that only UV Sterilizers help. Remember that the algae must pass by the light in order to affect it.
Dyes have been helpful to decrease the growth of algae. I have used blue in a larger watergarden to deter the growth of curly pond weed.