Algae are a sign of a healthy pond. The conditions favorable to string algae are limey water with a high pH. Run off from concrete slabs, cement, soil and even lime in tap water if you top off the pond frequently. Topping off the pond with tap water also feeds it the nitrates and the phosphates which are a great food source for algae. Phosphates also come from water running off of surrounding soil. String algae can grow at much lower temperatures than most of the competing plants in the pond and so it gets a head start on all of them.
The most natural form of treatment is to harvest as much string algae from your pond as possible. With persistence, the other pond plants will compete for the same nutrients and starve out the string algae provided the proper amount of pond plants are in place. The general rule of thumb is to cover 60%-75% of the water surface with aquatic plants such as floating pond plants and water lilies. This will help to reduce to photosynthesis capability of the string algae as well. There becomes a point where the most prevalent sources of food (sun-light, nitrate & phosphorous) are depleted and the string algae starves. Oxygenating/Submerged pond plants also compete with string algae for food.
If you don’t have the proper plant coverage, the products in this category in conjunction with products in the Routine Pond Maintenance Category can help get a handle on the problem of string algae.
To find products to help treat string algea buildup, check out Pond Maintenance > Water Treatments > Algae Control