Controlling String Algae and Green Water in Ponds

String algae and green water are the two most common pond algae problems. Learn what causes them, how to control each, and why balance beats eradication.

Two algae problems dominate garden ponds. String algae (also called blanketweed or filamentous algae) forms long green strands and dense mats on rocks, waterfalls and along the margins. Green water is a bloom of microscopic, free-floating single-celled algae that turns the whole pond a pea-soup green. Both are driven by the same root cause and respond best to restoring balance rather than chasing a quick kill.

What causes pond algae

The basis of the problem is an abundance of soluble nutrients in the water, which allows algae to grow rapidly and sustain large populations. In a pond these nutrients (nitrate and phosphate) come from fish waste, uneaten food and run-off, and they build up over time. Algae also need light, so sunlight on a new or unbalanced pond fuels growth. Filamentous algae starts on the bottom or attached to structures and floats up into surface mats sometimes called pond scum.

Controlling string algae

• Reduce nutrients: feed less, avoid overstocking and keep run-off out of the pond
• Remove strands manually by twisting them out on a stick or by hand; mechanical cutting tools also exist
• Add plants for shade and nutrient competition, and beneficial bacteria to consume waste
• Use barley straw, which is thought to release peroxides as it breaks down that suppress algae

Controlling green water

Because green-water algae float freely, the most effective control is a combination of a pond pump, an ultraviolet clarifier (UVC) and a pond filter: the UV light damages the algae cells so they clump and the filter removes them. Reducing nutrients and adding shade and plants tackles the underlying cause, while keeping the water moving and well filtered helps clear the bloom. A dedicated guide covers UV clarifier sizing and use.

Algaecides and the oxygen-crash risk

Chemical algaecides are a last resort. A large, sudden algae die-off is dangerous because decomposing algae consume oxygen, and dense algae already drive oxygen low in the early-morning hours, which stresses fish and in extreme cases causes fish kills. If you must treat a heavily affected pond, treat in sections and let each section decompose for about two weeks before treating the next, and aerate, especially at night, for several days after treatment to offset oxygen depletion.