How Can I Prevent Pond Algae?

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Not all algae is bad First, let's get our minds wrapped around this: Not all algea in ponds and lakes is bad. The problem comes when you have too much algae, or the wrong type of algae. In fact, the best treatment for pond algae, is algae! I know it sounds a little strange, but that is the way it works. What is pond algae When most people think of pond algae, they think of the type of algae commonly called pond scum, or blanket weed. The proper name for it is fillamentous algae. This is a type of algae that forms long fillaments or strings that then mat together to form a blanket on the surface of the water. This mat or blanket of algae makes fishing, boating, and other recreational use of a lake or pond difficult and distastefull. Pond scum also allows a perfect environment for mosquito production. In general, blanket weed in a lake is not a good thing for human use and interaction with the lake. Understanding pond algae To understand the best treatment for it, we need to understand where it starts, and how it starts. When we notice pond scum, we notice it on the top of the water, but that is not where it begins. Fillamentous algae starts at the bottom of your reservoir, and rises through the water profile as it gains bouyancy from the oxygen it produces. Eventually, enough oxygen is produced to lift it to the top of the water where it floats annoyingly. Clear water is the problem The problem is not that your pond or lake water is too murky, the proble m is that it is too clear. Again, I know that this is counter intuative, but bare with me. You have probably heard someone say "I can't see through muddy water." when they wanted you to move out of their line of site. The same principle applies here, because light penetrates straight through clear water, and because sunlight is needed for plant production, if the water is not clear, plant production is limited. If your water is too clear, you will have pond algae. The answer is not to go out and make the water muddy, the answer is to produce a single cell algae known as phytoplankton to tint the water, and prevent sunlight from reaching the bottom. So how do we prevent pond algae? Pond fertility is the answer. Check your pond or lakes water. If it is clear to a depth of more than 20 inches or so, it is too clear, and chances are, your fish are going hungry as well. A good fertilization program will solve both problems, and many others as well. This is how it works: When the proper amount of the proper fertilizer is introduced into the water, in the proper way, the single cell algae known as phytoplankton will reproduce rapidly in a process called algae bloom. This will continue as long as the water is fertile. The green color, or the golden brown coloring that this method produces will tint the water, preventing sunlight penetration, and preventing the growth of fillamentous algae. To learn how to do this see these pages, How To Fertilize A Pond Or Lake Lake Fertilization Lake Management Lake Weed Control Water Depth Secchi Depth and all the posts in this section of Home And Garden Press: Lake Management

Pond Water Color

Water coloring for weed and algae prevention

In order to prevent the growth of weeds and string algae, water in almost any type of pond or lake should not be perfectly clear. The way the water is colored is of major importance.

Clear water will not stay clear very long when exposed to the sun. Something will grow in the water, and it will probably not be what you want. To control this in farm ponds, fish ponds, lakes, or even in koi ponds and other backyard ponds, you will have to take some action. Prevention is better than trying to treat for slime, blanket weed, or submersed lake weeds later on. Getting out in front on this issue could save you a lot of money in treatment costs.

Pond water color good and bad

Not all pond water color is good. Turbid or muddy water isn’t good for any beneficial aquatic life, and the problem causing the turbidity should be solved before anything else can be done to improve water quality. This problem can occur for a variety of reasons, the most common of which is a bare watershed. Once the cause is dealt with, the water can be cleared using a flocking compound, and then the real work of getting the water properly tinted can begin.

The best pond water color is achieved with algae. Not string algae or pond scum, but the type of algae suspended in the water, known as phytoplankton.

Phytoplankton for pond water color

Phytoplankton is the best solution to most water clarity issues, it shades the water against sunlight, and uses nutrients which would otherwise be used by weeds and string algae, and it feeds zooplankton which feeds fish. There are several different types of phytoplankton.

Diatom algae

The type of phytoplankton present in the water is also important. There are many types, but perhaps the most productive is diatom plankton. These are the microscopic plants with silica exoskeletons which exist both in salt water and fresh water. Like phytoplankton in general, their numbers are diminishing, and have been for many years. Diatoms are very efficient in food production, and present a marked increase in zooplankton when they are present in large enough quantities. Diatom algae adds a golden brown tint to pond water.

Green algae

Green phytoplankton, have cellulose as their skeletons and produce increases zooplankton, but to a lesser degree than diatoms. Both produce color in the water, both remove excessive nutrients from the water, both produce oxygen, break down carbon dioxide, and raise dissolved oxygen levels, and both reduce weed and string algae production. Diatoms produce these desirable effects more efficiently, and great strides forward have been made in efficient means of producing it. Green phytoplankton produces similar results, but phosphorous is needed to produce and maintain this bloom, and phosphorus supplies are running low. Not all green algae are the same.

Dyes for pond water color

There are occasions where phytoplankton production may be difficult to acheive due to a variety of factors such as local water use restrictions. It is even possible to produce some types of fish without the presence of high concentrations of phytoplankton. Catfish respond well to regular feeding, and the introduction of fathead minnows into the environment, but for weed control, and for temperature control, some coloring of the water is needed. In such situations, and for decorative fish ponds, fountains, and other decorative garden type ponds, dyes may present the best possible alternative.

Pond Algae And Water Clarity

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The relationship between pond algae and water clarity can seem a little confusing for a pond owner. Clear water is not always good water, and one type of pond algae is the best prevention method for undesirable pond algae. There are many types of pond algae, some desirable, and some not.

If pond algae, of the pond scum or blanket weed type is forming a dense mat on your pond or small lake, there is about a 98% chance that the water beneath the mat is crystal clear. Check it out, and then come back, and I will tell you how I know this.

Back so soon?

It was clear wasn’t it? How do I know? Simple. Pond algae needs sunlight to grow. Pond algae needs soil to grow. For sunlight to reach the bottom of your pond, where pond algae gets it start in life, the water has to be clear. As the pond algae grows, some of the oxygen it produces by photosynthesis is deposited on the bottom of the mat of forming pond scum, and gives it the buoyancy to rise to the top of the pond. Submersed pond weeds, and blanket weed pond algae need clear water to produce, and in spite of all the thousands of products on the market that tout the virtues of clear water, and why their product will produce clear water, clear water is not desirable in a pond or lake where fish are producing.

Types of unclear water

Let’s make a distinction between types of unclear water. Do not confuse muddy or turbid water with the properly green colored water produced in a healthy pond or lake. Muddy water is not healthy water. In fact, water containing suspended soil particles is bad for the fish population. It is an irritant to fish, which causes them to feed less, fidget more, and often lose weight. It also prevents the natural photosynthesis process needed for the food chain. No, the water color needed for a healthy fish pond has a green tint, that will prevent sunlight from reaching more than 18 to 24 inches below the surface. There are products on the market that will color the water to prevent sunlight from reaching the bottom, but they have the same issue that makes muddy water undesirable. They suspend the food chain activity. So what is this magic color that prevents sunlight and provides the beginning of the food cycle?


Phytoplankton is a single cell algae that becomes suspended in lake and pond water, and is important for what it produces and what it prevents.

Oxygen production

Phytoplankton produces oxygen through photosynthesis which becomes available to aquatic life. Without oxygen in the water, your fish will die.

Shade and cooling

The phytoplankton type of pond algae provides shade for the water which helps to keep it cool. This is very important in the hotter parts of our country during the summer months.

Food production

  • Phytoplankton is the food source for the tiny creatures known as zoo-plankton
  • Zoo-plankton becomes food for insects
  • Insects become food for small fish
  • Small fish become food for large fish
  • Large fish become food for you.

Weed and pond algae prevention

The tinting effect of phytoplankton prevents sunlight from reaching the pond bottom, which prevents pond algae and submersed weed problems.

Producing phytoplankton

So, how can you produce this miracle pond product? First: If you have a heavy cover of lake weeds or pond algae, get rid of it.

Check the water color.

You can do this with a tin foil pie pan nailed to the end of a stick, although a more accurate measure can be attained using a “secchi disk” designed specially for this. Either way, if you can see the disk or pan at a depth greater than 18 to 24 inches, your pond is too clear to be healthy.

Test the waters pH

You can do this with a swimming pool test kit that you can purchase at most places that sell pool supplies. If the pH is between 8.6 and 6.8 you can go on to the next step. If it is lower than 6.8, the acidity is too high and will need to be changed before the next step. Agricultural limestone will correct the problem. I ton of agricultural limestone on one surface acre of water will raise the pH by 1 point on average. If you have a 1 acre pond, and the pH is 4.8, you will need at least 2 tons of limestone to bring your pond up to the minimum pH needed for healthy aquatic life. Do this before taking the next step.

Fertilize your pond

Fertilization will produce algae bloom, or phytoplankton in your pond. The right fertilizer is a fertilizer high in phosphorous and low in the other 2 elements. Phosphorous is the middle number in the 3 numbers listed on a bag of fertilizer. You need to add 40 lbs of 20% phosphorous, or 20 lbs of 40% phosphorous, or something similar in ratio per acre to initiate algae bloom. See also: How To Fertilize A Pond Or Lake

Pond Scum Control Advice


Filamentous algae, otherwise known as pond scum, is a nightmare for many pond and lake owners. The pond scum problem has a lot in common with other types of lake vegetation problems, and the cure for one will probably cure both. So, how do you get rid of pond scum? There are a number of different treatments, but the best method for dealing with pond scum is to prevent it from starting, and the method of prevention may come as a bit of a surprise.

Pond scum treatment

Pond scum, no matter where it ends up, starts at the bottom of the pond or lake. Pond scum is noticed most often on the surface of the lake or pond, but like emergent, or submersed lake weeds, it has its start on the lake bed. Pond scum treatments usually involve the use of a copper substance in liquid or granular form, or a contact weed killer like diquat. These products kill the algae, by punching tiny chemical holes in it, and the pond scum loses it's buoyancy and drops to the bottom. If you have this problem over a good portion of the lake or pond, it is best to treat only one third to one half of the lake at a time because the rotting vegetation requires oxygen to decompose, and it will take it from the water, which leaves less oxygen for your fish.

Pond scum removal

It is possible to physically remove pond scum from the surface of the water, but this process involves a good deal of manual labor in either skimming, or raking it. If you use copper sulfate or one of the other commercial herbicides labeled for pond scum or blanket weed control the algae will turn brown and sink to the bottom where it will degrade on it's own. Be aware that if you do not perform proper follow up procedures, the filamentous pond algae will return! Treatments of this type are sometimes needed, but such pond treatments are like the emergency room doctor after an auto accident. We are glad that he is there, but, it would have been better to have avoided the accident to begin with.

Preventing pond scum

Pond scum starts at the lake or pond bottom, it begins when sunlight penetrates to the bottom. Preventing light from reaching the bottom will prevent pond scum in the majority of cases. The prevention of pond scum, like the prevention of lake weeds requires doing something that will keep sunlight from reaching the lake bed. This involves water depth, and water clarity.

Adjusting water depth

There are two ways to change water depth. Raise the water level by adding water and maintaining it at a higher level, or removing soil from the bottom.

Raising the water level

Any attempt to raise the level of a spillway or overflow pipe should be preceded a thorough inspection  of the lake by someone who has the expertise to determine the success of such a project.

Removing soil from the lake bed

The other method for adjusting the depth of water in a pond or lake is to remove soil from the lake. This dredging process can be accomplished by draining the body of water and using earth moving equipment  to remove sediments, or by leaving the water, and hiring someone with specialized equipment built for the purpose. Either way, the project will be costly and time consuming. Ideally, there should be no area of the lake or pond that is less than 3 feet deep.

Adjusting water color

If water is too clear, then enough light can penetrate to produce filamentous algae. If sunlight can penetrate more than 24 inches, the water is too clear. There are colorants available which can do the job, but such colorants interfere with the natural food chain in the lake. The best means of producing color in a pond or lake is with tiny single cell algae known as plankton. Producing an "algae bloom" will reduce the depth to which sunlight can penetrate water, and prevent pond scum from becoming a problem. This is something that should be done even if your pond or lake is deep enough already. It will encourage the food chain and provide more healthy fish for your recreational fishing and for the table.

How to produce algae bloom

To start algae bloom, all that needs to be done is to add the proper amount of the proper fertilizer to the water. This should be done only after testing the water for proper pH, and amending the pH if needed. Once this is done, and the fertilizer added, the reaction will be swift. This should be followed by regular applications at 2 week intervals, and water clarity should be tested to be sure that it is working, and that water clarity is no more than 24 inches, and no less than 12 inches. If secchi depth is less than 10 to 12 inches, skip the next fertilizer application and re check the clarity periodically until it gets into the acceptable range before re starting the program.

Natural pond scum prevention with barley straw

There is evidence that pond scum can be prevented by using barley straw. The straw is submerged, or partially submerged in the water, and the decaying barley straw produces some chemical reaction that is not yet fully understood, but which seems to discourage filamentous algae production. It should be stated again, that this is not a treatment for existing pond scum, but another tool in our pond scum prevention kit. Remember that any algaecide, including barley straw is indiscriminate in pond algae destruction. When the pond scum is destroyed, there is a good chance that it will destroy the beneficial pond algae as well. Follow up is essential for for healthy ponds and lakes. For a paid consultation about dealing with your pond scum problem see:

Lake Consultation