For string algae and periphyton algae especially, reducing sunlight to your pool or pond can greatly inhibit the algae's ability to photosynthesize (thus limiting its growth). You can shade your water feature with trees, shade structures, stretched shade cloth, and/or non-toxic pond dyes. Achieving 50-70 percent shade is often recommended for maximum benefit.
Non-toxic pond dyes, tint the water just enough to block most of the sun's rays, making it harder for algae to grow. Adding dye, especially in early spring or when a pond in new, when biological activity is just beginning, is a great way to help keep your pond clean and clear of algae blooms.
Remove Excess Nutrients
Certain types of algae can grow with even the smallest amounts of nutrients, including decaying leaves, fish food and plant fertilizer. Phosphorus, one of the main nutrients required by algae, can be treated by some products, and is naturally absorbed by certain biological filter media like expanded shale (used in Natural Swimming Pool/Ponds by Total Habitat). Proper water movement and biological filtration will help reduce excess nutrients as well as a thorough Spring Cleaning.
Add a UV Sterilizer/Clarifier
Planktonic algae can be reduced or even eliminated by adding a UV unit to your plumbing system that is capable of at least 22mj/cm2. Water flows through the UV Sterilizer chamber where it (and any microorganisms floating in it) are exposed to a super bright UV light. This UV light kills the organisms or inhibits their ability to reproduce. Be sure to plumb your in-line UV Sterilizer after your pump (on the discharge side), preferably after a mechanical filter like a sand or cartridge filter. Typically, you will need to turn over the entire pond volume at least 4-6 times per day through the UV unit at the required flow rate (faster than recommended flow will result in less than necessary exposure time).