There are a few common types of algae that grow in water bodies, and string algae are one of them. These organisms are filamentous and form thin hair-like strands in pond water. These algae aren't a danger to fish or other aquatic species in small amounts because young fish species use string algae as a food source. There is a danger however if string algae notably increases in size. If string algae grows out of control it will deplete the oxygen levels and increase the biological and chemical oxygen demand. As a result, the water quality will be degraded.
Aqua Gardening has many products for treating string algae. Splosht Fishpond products will reduce the excessive concentrations of nitrates, phosphates and ammonia. API Pond Algaefix will control string algae blooms and help to clear the water. PondMAX Complete Water Treatment can be used to kill off algae and keep the pond water clear. Increasing oxygen levels via proper aeration is also required when treating pond water for string algae, please see the pond aeration category for air pumps to speed up the elimination process.