How to Clear Green Pond Water (Best Treatments)

Pond Informer is supported by it’s readers. We may earn commission at no extra cost to you if you buy through a link on this page. As an Amazon Associate we earn from qualifying purchases.

Help Spread Pond Keeping Knowledge!

How to Clear Green Water in Ponds & Lakes 2020 (Top Treatments)

Green pond water is the bane of many pond owners, and understandably so, as it can harm or even kill both fish and local vegetation if left uncontrolled. Water that is tinged green (and that may or may not also have floating bits or entire mats of algae) is ultimately caused by nutrient imbalances, such as a surplus of nitrogen or phosphorous, that fuel the rampant overgrowth of various types of algae. Excess nutrients can be attributed to a variety of potential culprits – grass clippings left to float in the water, fertilizer run-off, having too many fish and thus too much fish waste, lack of adequate aeration or filtration, or not enough plants to properly cycle the nutrients.

Is Green Pond Water Dangerous? (Yes, and no)

cyanobacteria is very dangerous to ponds
Often confused with green water algae, blue-green algae is more dangerous if left uncontrolled. Photo credit:

In both garden ponds and naturally occurring lakes, there are a few algae species in particular that are known to turn water green if left unchecked. One of the most well-known (and perhaps the most dangerous) is cyanobacteria, or blue-green algae. As mentioned in a previous article, blue-green algae is not algae at all, but rather bacteria as its scientific name implies. Nonetheless, it can behave similarly to algae in water – thriving on superfluous nutrients, particularly in warm, still, poorly aerated water, and turning the water various shades of green or blue. Ultimately, cyanobacteria can render the water hypoxic, killing any inhabitants once a full bloom has established.

In comparison, regular green algae species (from the family chlorophyta, containing over 7,000 different species) are the most common (and arguably the least harmful) offenders. Green algae is at the base of the aquatic food web, providing an invaluable source of food to insects, waterfowl, fish, frogs, and others.

a pond with a healthy amount of algae
A slight tint of green water is considered healthy, but a deeper green color often indicates a nutrient imbalance.

When green algae is involved, discolored water may not actually be a sign of declining ecosystem health – green algae species are numerous and occur naturally in virtually every body of water around the world, so simply conduct a water test. If the results are within normal, healthy parameters, you just have some green algae. Warmer temperatures and an increase in sunlight in the spring and summer can cause these guys to temporarily ramp up their numbers (until the pond/lake, its inhabitants, and organic ecosystem functions naturally bring their population down), leading to greenish water.  However,  as a pond owner you need to keep an eye on your nutrient and oxygen levels nonetheless, as nutrient dense water can cause these species to bloom out of control as well, thus increasing the likelihood of an algae blooms that will lead to a sharp reduction in dissolved oxygen and an increase in waste substances. If your water isn’t just tinged slightly green (which can be considered normal so long as it doesn’t persist for an extended period of time) but rather is very green and opaque, this is certainly cause for concern and you should take actions to control the green algae!

The Best Green Pond Water Treatment Methods

1 ) UV Clarification

Recommended for:  Green Water Algae.

UV clarifier to treat pond algaeA superbly effective method for green water algae, ultraviolet clarification methods are even used by many water treatment plants in the U.S. and Europe to kill off any unwanted organisms that managed to slip through the rest of the cleaning process. Non-industry-sized UV filters typically kill algae within one to two weeks, in addition to killing off any harmful bacteria. However, you should be aware that while these clarifiers will make your pond look nice and clean, they don’t actually filter the water. You should still regularly perform water quality tests to ensure proper pH, oxygen, and nutrient levels.

While efficient and successful, this method does have a few negatives attached. First, it likely won’t be particularly effective for an entire lake, as it’s simply too large of an area, so using it for standard fish ponds or aquariums is best. Also, depending on the filter and the size, they can be fairly pricey. Getting them to work properly also takes a bit of finesse – if you pump the water through too quickly, the clarifier won’t be able to actually kill the algae; too slow, and the algae will simply continue to multiply despite the clarifier. Furthermore, UV clarifiers will only kill floating algae, not other types such as string algae or blanket algae. If you’re dealing with multiple types of algae, a combination of  UV clarifier and vacuum is a great combination for garden pond usage.

2) Lake & Pond Water Dyes

Recommended for:  All Types of Algae.

blue pond dye treats pond algaeAdding dye to your water can help to reduce the amount of sunlight that penetrates the surface (also known as the photic zone). By doing this, the water temperature is lowered along with the amount of sunlight, both factors that will help to lessen and deter algae growth. Dyes, particularly blue dye, are useful against floating single-cell algae, filamentous algae, and algae that congregate to form mats.

Utilizing dye is more of a pre-emptive measure, however, as it won’t by any means kill off a substantial algae population or a bad bloom. An added bonus is that dye makes it more difficult for predators to see your fish, and is considered safe for humans, animals, and plants. It’ll also deter the growth of submerged plants such as water milfoil, pondweeds, and elodea – this can be either a pro or a con, depending upon your perspective and goals. The main downside is that the dye will wear off after a month or so, and continually purchasing it can really add up financially. In its concentrated form, it will stain just about anything that it comes in contact with so you’ll need to store it carefully and use protective gloves and clothing.

3) Air Pumps & Aeration

Recommended for:  All Types of Algae.

air pumps aerate water and reduce algaeProper air flow helps to achieve and maintain necessary dissolved oxygen levels and keeps water moving to prevent stagnation. Still, stratified, hypoxic water is a hotbed for algae establishment and growth. An air pump is something you should incorporate into your pond or lake anyway, regardless of algae, as the majority of freshwater fish species need dissolved oxygen levels to be 6 parts per million (or 6%) at a minimum (though 7 ppm is better), and air pumps or aerators in combination with aquatic vegetation and keeping algae under control help accomplish this. Once oxygen levels drop to at or below 3 ppm, most fish will begin to die.

You can also use an air compressor to push air up from the bottom of the water body; this option tends to use less energy than standard air pumps, but can present a higher initial cost (though this could even out over time with the energy saved). A potential drawback of air pumps and compressors is that they can cause shoreline erosion (mainly in smaller water bodies like ponds), particularly if you don’t have any plants or a liner to hold the sediments in place, such as in natural lakes.

4) Competing Plant Species

Recommended for:  All Types of Algae.

plant species like hornwort outcompete algaeA very useful (and natural) method for controlling algae growth and getting rid of murky, green water is introducing native aquatic plants. These plants thrive on the same thing as algae: nutrients. Add them into your pond, and they’ll cycle through nutrients, release oxygen, and help shade the pond (thus providing temperature regulation and decreasing the amount of sunlight that penetrates the photic (or sunlight) zone – all things that will deter algae growth. There really isn’t a downside to this method; you’ll just have to take some time to figure out which plant species work best for your location and fish species.

5) Rakes and Vacuums

Recommended for:  String & Filamentous Alage.

pond vacuum to clear string algaeThis approach is rather self-explanatory – you can utilize rakes to remove filamentous algae and algae mats, and vacuums to remove free-floating algae. If you’re going to go the manual route, you should use both of these means. First, rake out any obvious larger areas of clumped algae, as these could clog a vacuum. Then, use a vacuum to help get the smaller, suspended stuff out. However, be aware that some vacuums may also pick up and damage plants and sediments, so use carefully. Manual methods like this are more cost effective, but are just that: manual. They’ll take some time and work to do the job, and you’ll have to repeat it regularly to ensure your pond remains healthy. Furthermore, rakes and vacuums work best for smaller areas like ponds, as using them on an entire lake would be immensely time consuming and would not be as effective in removing all algae as other methods.

6) Barley Straws

Recommended for:  All Types of Algae.

barley straw exudes chemicals that kill algaeBarley straws are a simple and economic way to control algae. Because barley straw does not actually kill algae but rather impedes the growth of any new algae, it’s best to place it early in the year before algae begins to colonize (usually in the fall or spring). When exposed to sunlight, barley straws exude a chemical that stops the growth of algae (but again, will not kill off any existing algae) and will remain effective for as long as 6 months. This method works well for both ponds and lakes, but you’ll need to get the timing and quantity right – you’ll need about 1 pound of straw for every 2,000 gallons of water, and the straw will need to be broken apart and fluffed up a bit so that the water can still flow freely through it and remain oxygenated despite the decaying straw.

Another thing to note: in some cases, using barley straw has increased the growth and reproduction rate of plants species present, as there is less algae for them to compete with.

7) Algae Eating Fish

Recommended for:  String & Filamentous Alage.

koi eat some algae in ponds

As discussed in the “Which Fish Will Eat Algae in Ponds” article, there are numerous fish species that are quite efficient at consuming algae. It is important to note that fish likely won’t entirely eliminate algae, particularly if you have enough to cause green water, but in combination with other methods they will help to keep the algae at a more stable and manageable level. Among the best algae eaters are: plecos, weather loach, Siamese algae eater, mollies and guppies, otocinclus catfish, and the Chinese high fin banded shark. Koi and goldfish will also munch on algae, but it’s not a staple of their diet so you’ll need to incorporate other algae eaters into the water as well, or utilize additional algae control approaches.

Alternatively, you can also reduce the amount of fish in your pond, as too many fish will produce more waste than can be properly cycled, thus fuelling algae growth. You should also make sure that you are not feeding your fish more than necessary (only feed them as much as they’ll eat in a 5 minute span); any uneaten food will contribute to nutrient imbalances and help algae prosper.

What About Algaecides for Green Water?

The use of algaecide will depend on your personal preferences, type/size of pond, and the extent of your algae issue. As indicated in numerous other articles, anything designed to kill something has the potential to have adverse impacts on other things as well, so these should really only be used as a last option – especially if you have fish. Algaecides are obviously great at killing off existing algae, preventing future growth, and clearing up your water – often all within 24 hours – but not all are created equal. We cover some common options below:-

1) Copper Sulfate

Copper sulfate is an affordable option as far as algaecides go; a 5 pound bag is enough to treat about 20,000 square feet. It comes in a granulated form, so it’ll dissolve before your fish have a chance to attempt to eat it. Though an effective algae killer, it does have some drawbacks. Copper sulfate can settle into the sediment, where it continually builds up and leeches into the water and can become toxic over time, harming your fish, snails, as well as beneficial bacteria. To avoid this, use the minimum amount necessary and do not use it frequently.

2) Cutrine Plus

cutrine plus kills most types of algae in pondsEPA approved, Cutrine Plus is available in both a liquid and granular form. Its effectiveness stems from its ability to kill off many types of algae: filamentous, planktonic, and chara. However, the label states that its best suited for eliminated surface algae, such as surface blooms and mats. With this in mind, it may not be the best option for entirely clearing up green water. It’s considered safer than copper sulfate and persists in the water longer, so you’ll have to treat less often, though it does also contain copper and can become toxic to your fish over time.

3) Green Clean (natural)

Unlike most algaecides, including those listed above, Green Clean is an entirely natural algae killer that is specifically designed to be used in ponds and lakes with goldfish, koi, and catfish. It’s available in both liquid and granular forms, though the granules can burn plants if in direct contact, so be careful with dispersal or use the liquid version if you have many plants. The label does warn that this product is toxic to birds and bees if ingested, so it’s best to apply Green Clean away from pollinating areas on a calm day (so that wind won’t carry it to other areas) and early in the season before pollinators and algae are particularly active. Despite this, Green Clean is one of the safer options for your fish and plants, and works very quickly – you’ll notice almost immediate bubbles as well as clearer water within an hour, according to the label.

2 thoughts on “How to Clear Green Pond Water (Best Treatments)”

  1. Great information. I live in South Florida and for several years, I have been using GreenClean Algaecide (powder) in my 1100 gallon pond which has 9 butterfly Koi . Being in a hot climate it was almost impossible to keep my pond water from turning green. I was reading your POND INFORMER website, “How to Clear Green Water in Ponds & Lakes 2020 (Top Treatments)” WOW, not only did it make the pond water crystal clear, it also took all the algae off of the water fall rocks. FANTASTIC PRODUCT.

    • Hi Butch,

      Thanks so much for reading and for the kind feedback! We’re so glad that the information was helpful to you, and that you’re enjoying the benefits of a clean pond! Excess algae growth can be difficult to get under control, particularly in a subtropical climate like Florida, so we’re happy that the product worked for you.


Leave a Comment

This site uses Akismet to reduce spam. Learn how your comment data is processed.