Algae and green water2026-10-04

How to clear green pond water, and why it keeps coming back

Green water is single-celled algae floating in the water, and it clears for good only when the pond holds fewer nutrients and gets less light than the algae need. A UV clarifier sized for the pond usually clears it within days by killing the cells as they pass the lamp. It keeps coming back because the cause stays behind: too many fish or too much food, too few plants, muck on the bottom, and in some towns phosphate in the tap water. A new pond going green for its first few weeks is normal and passes on its own.

Single-cell algae and string algae are different problems

Penn State Extension says "Planktonic algae floats in the water column" and "causes the water to look green, brown, or reddish in color." String algae (filamentous algae) grows in strands attached to rock and liner. The Pond Guy's own UV article says UV clarifiers work "by destroying the ultrafine planktonic algae," but "they do nothing to combat the filamentous algae." This article covers the floating kind. String algae on rocks and waterfalls has its own article.

What the authoritative sources say

On causes, Clemson's Home and Garden Information Center says "The general cause of algae in ponds is excess nutrients from either the overstocking or overfeeding of fish, or the lack of nutrient uptake by plants caused by planting too few plants in a pond." In a new water garden, it says, "a green water condition will exist for approximately three weeks" and then clears. If your pond is that young, read our article on new pond syndrome first, because ammonia is the bigger risk at that stage. Oklahoma State's water garden fact sheet advises to "stock small numbers of fish and limit or totally skip the use of fish feed."

On plants, the published targets cluster around half the surface:

Plant cover targets for green water, and what they mean in square feet
Source	What it says	3,000 gal pond, 160 sq ft of water	10,000 gal pond, 535 sq ft of water
Clemson HGIC	"as much as 50 percent of the water surface"	up to 80 sq ft	up to 267 sq ft
The Pond Guy	"shading 40-60% of your pond's surface area"	64 to 96 sq ft	214 to 321 sq ft
Aquascape (search snippet)	"approximately two-thirds of the water's surface"	107 sq ft	357 sq ft

The 3,000 gallon row is a 16 by 10 foot pond, and the 10,000 gallon row assumes an average depth of 2.5 feet. Webb's Water Gardens, a retailer, warns on its UV page that "your pond might retain a greenish tint unless you have at least 50% plant coverage or your pond is in partial shade."

On UV, Aquascape's UltraKlear manual rates its own units by pond size and by flow: the 14 watt model for 1,000 gallons at no more than 500 GPH, the 28 watt for 2,500 gallons at 1,000 GPH, and the 55 watt for 5,000 gallons at 1,500 GPH. For the two larger models that is about 11 watts per 1,000 gallons, with a flow limit that matters as much as the watts. Our UV, filter and aeration sizing article compares makers' rules and explains the bypass. The Pond Guy says bulbs "should be replaced yearly," even if they still light.

On beneficial bacteria, The Pond Guy says its products "consume the nutrients and other organic material in your pond." We found no controlled study of bacteria products in ornamental ponds. The nearest is Queiroz and Boyd at Auburn University (1998), who dosed catfish ponds three times a week from May to October and found "few significant differences" in water quality between treated and untreated ponds, and who note that earlier trials also "failed to demonstrate differences in water quality." Catfish ponds may behave differently from a koi pond, but this is the closest test we found.

On algaecides, the Southern Regional Aquaculture Center's herbicide fact sheet (SRAC 361) gives the main warnings. "Fish species such as trout and koi are particularly sensitive to copper. However, most fish kills that occur after copper sulfate treatment are caused by a massive algae kill and the subsequent oxygen depletion due to decomposition." It says "In water with an alkalinity ≤ 50 ppm, the rate of copper sulfate needed to control algae can be toxic to fish," and that "Treating only one-fourth to one-third of the total surface acreage of a pond at one time can minimize the risk." It adds that "Fish should be observed for 1 week after treatment." Texas A&M's AquaPlant says to let each treated section "decompose for about two weeks before treating another section," and Clemson says that with copper, "aeration must be available."

What contractors and experienced owners do

Owners split on UV. On Garden Pond Forum in 2021, jdogfunk100 wrote that after 12 years with a 3,500 gallon pond, "I finally retrofitted a UV light system and believe it not, I finally have clear water." brokensword replied, "Killing the free floating algae won't stop it from coming back if you have not found the source. Typically, overstocked and under-filtered, over-feeding, and a lot of debris decaying on the bottom." In 2019, mgmine wrote of his UV, "I turn it on when I open the pond and within a week my pond is clear."

Others wait it out. Bluerooster, with about 3,000 gallons, spent three years on "all kinds of additives," filters, plants and water changes, then stopped in the fourth: "Now that I've just left it to it's own devices, the water is reasonably clear." In 2021, cas wrote, "every spring I have cloudy green water, but in a few weeks, it clears up on it's own."

The cautionary case is Vasi in 2021, with "almost 10000 liters" in full sun, "a very good filter and 24w UV light," and few plants. Three algae products in a row did "not work not at all," the koi turned "a little clampy," and Vasi changed all the water, expecting it to be green again within two days.

Ozponds, an Australian pond site, says "Topping up the pond with town water can introduce additional nutrients, particularly phosphate." Pond Talk, The Pond Guy's forum, blames a UV that stops working on fast flow or an old bulb: "Twelve to fourteen months of operational life is common."

Where the sources differ, and why water changes alone rarely work

Plant cover targets range from 40 percent to two-thirds, and none of the sources says how it arrived at its figure, so aim for the middle. Practitioners call UV either the whole answer or a crutch, and both are right: a UV kills the cells that pass it but leaves the nutrients, so the water stays clear only while it runs and the bulb is fresh.

Water changes are the weakest fix. A change removes the same share of the algae and of the dissolved nutrients, and the cells left behind keep multiplying on what remains, on the muck and on the next feeding.

What a water change leaves behind in a 5,000 gallon pond
Change each time	Gallons replaced	Algae and dissolved nutrients left after one change	Left after three changes
10 percent	500	90 percent	72.9 percent
20 percent	1,000	80 percent	51.2 percent
30 percent	1,500	70 percent	34.3 percent
50 percent	2,500	50 percent	12.5 percent

The table assumes the new water carries no phosphate, and many towns' water does. The EPA's corrosion control guidance says utilities dosing orthophosphate to control lead "target residual concentrations of 0.33 to 1.0 mg/L as P (1.0 to 3.0 mg/L as PO4) at the tap." That is 330 to 1,000 micrograms per liter. The University of Florida's lake classification puts a lake in its most productive class, hypereutrophic, at "total phosphorus more than 100 micrograms per liter." So in a town that doses phosphate, the water used for a change can carry 3.3 to 10 times that much. Ask your water utility whether it adds orthophosphate before relying on changes.

One number circulates without a basis. Ozponds gives a hydrogen peroxide dose of "1/2 a cup, per 100 US gallons" of the 3 percent household kind. We found no extension or label source for a household peroxide dose in a fish pond, so it is left out of the steps below.

What to do

If the pond is only weeks old, test ammonia and nitrite and let the bloom run its course.
Get the volume and the surface area. The volume sizes the UV and any dose, and the surface area sets the plant target.
Feed less and count the fish against the volume.
Add plants until about half the surface is covered or shaded. Water lilies cover the surface, and marginal plants on the shelves take up nutrients too.
Remove the muck from the bottom and clean the skimmer and filter.
Fit a UV sized by watts and by flow, with a bypass if the pump moves more than the unit's rated maximum, and replace the bulb every year.
Use an algaecide only as a last resort. Check the label for koi, test alkalinity first if it contains copper, run an aerator, treat a quarter to a third of the pond at a time, and watch the fish for a week.
Do not chase green water with large water changes, and do not stack one product on another.

The PondTools volume and water change calculators

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. The PondTools water change calculator (also standalone) takes the pond volume, the current reading, the target reading, what the replacement water reads and the percent replaced each time. Give it a phosphate reading for the pond and for your tap, and it shows whether changes can lower the phosphate at all. A pond traced in the PondTools app fills in the volume and gives the surface area for the plant target.