Algae control2026-10-04

How to get rid of string algae on a waterfall and stream

String algae grows thickest on a waterfall because the falls give it sunlit, wet rock and push a constant supply of dissolved nutrients past every strand, and in summer the water is warm as well. To clear it, brush off what you can, then turn the pump off, dust what is left with a granular sodium percarbonate algaecide at the label rate for your pond's real volume, and skim out whatever lifts off. It grows back unless the pond carries fewer nutrients. Barley straw has weak evidence against string algae, and copper is the treatment most likely to hurt koi.

Why string algae grows on the falls

String algae (filamentous algae) is anchored to rock and liner, so it never passes through a UV clarifier the way the floating cells of green water do. Our green water article covers the floating kind. The Pond Guy says "All that green nuisance needs to thrive is the right amount of nutrients and sunlight," and that it grows "Even in the cold of winter."

The water on the falls and in the stream is shallow, so the rock gets full light, and the current helps too. Horner and Welch measured algae on rocks in streams and found that "a velocity increase up to ~50 cm∙s⁻¹ enhanced periphytic algae accumulation when orthophosphate-phosphorus concentration exceeded 40-50 μg∙L⁻¹." Moving water strips away the thin, depleted layer around each strand and brings fresh nutrients, so in rich water a strand in the current outgrows one in still water. At lower phosphorus they found the current slowed growth. A waterfall usually runs out of a biological filter, which turns fish waste into nitrate, so the water reaching the falls carries exactly what the algae use.

What the labels and extension sources say

The granular products sold for waterfalls are sodium carbonate peroxyhydrate, also called sodium percarbonate, which releases hydrogen peroxide when wet and bleaches the algae on contact. Three labels give these rates for string algae:

Label rates for string algae, three sodium percarbonate products
Product and label	Active ingredient	1,000 gallons	5,000 gallons	Pump off for
EcoBlast, Aquascape (2018 label)	42.50 percent	1 cup	5 cups	10 to 20 minutes
GreenClean Granular, BioSafe Systems	42.50 percent	1 cup	5 cups	45 minutes
CrystalClear AlgaeOff, Airmax (sold by The Pond Guy)	50 percent	1/2 cup for heavy growth, 5 teaspoons for light	2.5 cups for heavy growth, 1/2 cup for light	20 minutes

All three labels say 2 cups weigh 1 pound, and all three say to allow 48 hours between treatments. EcoBlast and GreenClean say the product "can raise pH levels up to 1 unit" and to "Treat only a half or a third of any large body of water at a time." The EcoBlast label says to skim the dead algae, because "Allowing dead organics to sink and decay will provide a food source and additional nutrients that stimulate algae re-growth," and warns that "Non-target plants will suffer contact burn if undiluted granules are accidentally spilled on them." The labels call the product toxic to birds, and AlgaeOff says not to treat the margins "when amphibian eggs or young (e.g. tadpoles) are present." EcoBlast is labeled "For use in water features containing live fish and plants," but none of the three gives a tested fish limit. A newer EcoBlast listing gives 85 percent active ingredient, so read the label on the tub you buy.

On fish, a US Geological Survey study by Rach and others found that "the toxicity of hydrogen peroxide increased for all species as water temperature increased," and in rainbow trout "larger fish were more sensitive." Koi were not tested, but warm water and big fish describe a koi pond in July.

Barley straw has the weakest record. Purdue's Carole Lembi "often noticed an increase in mat-forming algae" in barley trials, and at Iowa State "The mat-forming algae were not reduced in the barley-treated enclosures." University of Idaho Extension sums up the US work: clarity improves "but thus far evidence suggests no affect on filamentous algae." Rutgers says "It does not kill existing algae," and Idaho warns that more than 450 pounds per surface acre "may cause an oxygen depletion resulting in a fish kill."

Copper kills string algae, and it is the riskiest choice with koi. The Southern Regional Aquaculture Center says "Fish species such as trout and koi are particularly sensitive to copper." Aquascape's IonGen releases copper ions from a probe. Tucker and others (Veterinary Record Open, 2020) examined a koi die-off in Raleigh, North Carolina, after an IonGen was added, found copper that "exceeded those associated with toxicity in both hard and soft water," and concluded that "ion generators might not be safe for fish." Texas A&M's AquaPlant adds that any algae kill risks "an oxygen depletion after the treatment caused by the decomposition of the dead plant material."

What contractors and experienced owners do

Most owners pull it by hand first. On Garden Pond Forum, LarryM fixed "a plain old cheap toilet bowl brush" to "an old broom handle." hobie61, with 2,500 gallons, added "a 40w UV light this spring" and still cleans the falls by hand "At least once a week," and Lisak1 replied that "UV light does nothing for string algae."

Many use peroxide with the pump off. Angel wrote, "Shut of stream pour peroxide on string wait 30 mins in sun brush off." sissy found the algae "turns white and floats away but filters get clogged fast." mrtrumpet buys plain sodium percarbonate instead of GreenClean, will "measure about 1/2 cup then dust all the areas with algae," and warned, "Careful for birds, though." Lisak1 hand-pulls before sprinkling and said that "in year five we haven't needed to use it at all." texmaster cut the light instead: "I got a shade for mine and it cut it 80%."

Some leave a little. Smaug calls it a "secondary filter system." On a TEAM Aquascape tour of the Aqualand ponds, the presenter calls it "a little bit of string algae out there which is normal and healthy" and says "the fish eat it." Splash Supply Company suggests you "Unplug your waterfall for 24 hours" so the rocks dry in the sun. That advice is written for pondless waterfalls. In a koi pond the falls usually carry the filter, so a day off is a day without filtration.

Where the sources differ

The labels disagree on dose. EcoBlast and GreenClean put 1 cup, or half a pound, of a 42.50 percent product into 1,000 gallons, about 0.21 pounds of active ingredient. AlgaeOff's heavy rate is a quarter pound of a 50 percent product, about 0.125 pounds. The Aquascape and BioSafe rate is about 1.7 times as strong, so do not swap rates between brands.

Household 3 percent hydrogen peroxide has no label for fish ponds. Ozponds gives "1/2 a cup, per 100 US gallons," which is 5 cups, about 1.2 liters, per 1,000 gallons. That carries about 35 grams of hydrogen peroxide, or about 9.4 milligrams per liter if it mixed through the whole pond. The EcoBlast label says its product "is equivalent to 13.80% Hydrogen Dioxide by weight," so 1 cup per 1,000 gallons works out to about 8.3 milligrams per liter, and AlgaeOff's heavy rate to about 4.9. That is our arithmetic for comparison, and no source gives it as a dose. Poured on the falls with the pump off, the strength on the rock is far higher than the whole-pond figure.

Barley rates disagree with each other too:

Barley straw rates for two ponds, both 2.5 feet deep on average
Rate	3,000 gallons, 160 sq ft of water	5,000 gallons, 267 sq ft of water
Rutgers, "1 pound of straw per 2000 gallons"	1.5 lb	2.5 lb
Purdue and Idaho, 225 pounds per surface acre	0.83 lb	1.38 lb
Idaho's oxygen ceiling, 450 pounds per surface acre	1.65 lb	2.76 lb

For ponds of this depth, the per-gallon rate comes close to the oxygen ceiling.

The most common error is the volume. If you dose for a guessed 5,000 gallons and the pond holds 3,500, the 5 cups of EcoBlast work out to about 1.43 cups per 1,000 gallons, which is 43 percent over the label rate.

What to do

Measure the pond's volume, and work the dose from the label of the brand you bought.
Brush or pull out as much string algae as you can and take it out of the pond.
Test pH. If it is already high, treat a smaller area, since the granules can raise it by up to one unit.
Early on a sunny morning, turn the pump off and dust the granules on the falls and stream at the label rate. Keep them off plant leaves and away from margins where frogs are breeding.
Restart the pump after the time on that label, skim the bleached algae, and clean the skimmer and filter pads.
Wait at least 48 hours before treating again. If you use bacteria, the EcoBlast label says to add the algaecide first and wait several hours.
Run an aerator after a large treatment and watch for fish gasping at the surface.
Cut the nutrients: add plants along the stream and falls, feed less, and thin the fish if the pond is overstocked.
Do not add copper or an ion generator to a koi pond without a copper test kit, do not stack products, and do not dose above the label rate.

The PondTools volume calculator

Every rate above is per gallon, so the volume is the number to get right. The PondTools volume calculator (also standalone) takes length, width and average depth in feet and a shape (rectangular, circular or oval, or irregular) and returns gallons. For an irregular pond, the PondTools app traces the outline from a satellite or drone photo and fills in the volume and surface area, so the dose and any barley rate come from the pond you actually built.