Algae Control 101 - What Causes Algae and How Do you Control It?

Algae control is probably the most common problem pond owners face year in and year out. It can appear virtually overnight and cause nightmares for months if not treated properly. This article will serve as a guide to help pond owners better understand algae and offer safe methods for both short-term and long-term algae control.

Types of Algae

The two most common forms of algae that ponds experience are planktonic algae and filamentous algae.

Planktonic algae (otherwise known as green water algae or pea soup algae) are microscopic, free floating algae the give ponds their clear green color. A normal population of planktonic algae is mandatory for a healthy pond, as they are the base of the food chain and essential for the health of the other aquatic life. When planktonic algae start to bloom and become too prevalent, they will give ponds a pea soup coloration. This usually takes place in summer months or upon the first significant warming trend of the season

Filamentous algae, often called pond scum, pond moss, string algae or hair algae, begins growing on the bottom of ponds on surfaces like rocks and logs and resemble green fur. As the clumps grow, they break loose from the bottom and float to the top, causing ugly green mats on the pond surface. Filamentous algae begin growing in the early spring and are first noticed around the edges of the pond in shallow water. It has little redeemable value to a pond and can even ruin a recreation pond during the summer.

Algae Problems

The primary complaint most pond owners have about algae is how it tarnishes the appearance their pond. Everyone wants to look at his or her pond and see crystal clear water, so murky green water or stringy green mats can be a major disappointment. However, it is what algae can do to the quality of the water that should be the bigger concern.

Algae in moderation is good for a pond, but when it becomes over abundant, it can cause severe water quality problems. Like other plants, algae grows through photosynthesis using sunlight and carbon dioxide and releasing oxygen. But at night, algae reverses this process and consumes oxygen to continue growing. This is why you can literally go to bed and wake up with a pond full of algae. This rapid growth and continued utilization of oxygen at night can result in dangerously low dissolved oxygen levels, especially early in the morning, which can result in fish kills.

Heavy algae growth will also negatively effect irrigation, should your pond be used for that, particularly in the case of string algae (filamentous). String algae will collect and clump up in the irrigation lines causing clogs in the system that must be manually cleared. This type of unplanned maintenance can add hours and cost into an irrigation system.

What Causes Algae to Grow?

Algae blooms are caused by three factors: excess nutrients, too much sunlight and low oxygen levels.

So where do the nutrients come from? Nutrients come from a variety of sources, most notable fish and animal waste, fish food and dead/decaying vegetation. Other sources would include fertilizer run-off and run-off from nearby farms and pastures. All of these sources provide much needed nitrogen and phosphorus that algae need to bloom and grow.

Ponds that sit in direct sunlight or have very few aquatic plants are also at risk of suffering from algae problems. As noted earlier, algae photosynthesize their food and sunlight is required. This is why generally the most and worst algae problems occur in the summer when the days are longest.

Finally, ponds that experience poor oxygen levels will also suffer from algae control issues. When there are high levels of waste in ponds that are stagnant and still, oxygen levels can be depleted due to the amount of oxygen needed to break down the waste. Algae blooms come and go, but when there is little dissolved oxygen in the water, dying algae will feed new cycles of living algae because there is not enough oxygen to break down the dead vegetation before it can fuel more growth.

Methods of Algae Control

First of all, algae should not be looked upon as a problem. It is merely a symptom and a sign that there are other problems and that your pond is out of balance. What it means is that one or more of the causes above are the true problems.

There are both short-term and long-term approaches to controlling algae. The short-term approach involves some sort of algaecide to quickly kill off the algae, which in most cases works. However, this in itself can cause problems. Some algaecides such as the copper-based ones can be harmful to fish if the directions are not followed precisely. However there are algaecides like GreenClean and GreenClean Pro that are non-copper based and can provide a quick, safe algae kill. As mentioned earlier, dead or decaying vegetation is a nutrient source for algae. So when you simply kill the algae and leave it to degrade on its own, chances are it will just fuel re-growth. Another problem with a quick kill with algaecides is that you run the risk of a fish kill. Killing off too much algae too fast can cause an oxygen depletion as the pond has to work extra hard to break down all of the dead waste. In scenarios where ponds have extreme algae problems it is recommended to kill of the algae in stages so that there is not too much decaying vegetation the pond has to contend with.

The ideal approach for using an algaecide for quick algae control would be to use the algaecide as a pre-cursor to the long-term approach discussed below.

The long-term approach involves utilizing methods that control the sources of algae nutrients, sunlight and low dissolved oxygen. As the name suggest, this type of control does not work as quickly, but in the long run it is safer and more cost effective.

Nutrient Control

There are a few ways to control the nutrient load,one is through manually removing floating debris and algae with various rakes, nets, brushes and skimmers. String algae can be pulled from a pond by simply twirling a rake or brush in the middle of a large algae mass. Additionally, being able to pull leaves, grass clippings and other floating debris from the surface will also be a big help.

In the case of small ponds like koi ponds, watergardens and backyard ponds, filtration becomes very important in managing nutrients. A good pond filter will offer both mechanical and biological filtration to help handle and remove fish waste. Some larger pond filters will also house a UV sterilizer which can be extremely effective in killing planktonic algae, but will not help with string algae. Generally, a larger pond filter will allow your pond to hold more fish, however this should not be an open invitation to overload your pond with fish. Effective nutrient control for small fish ponds begins with not overpopulating the pond. Many people subscribe to the rule of one 6" fish per 100 gallons.

One more way to help against nutrient overload, particularly in the area of run-off is to construct some sort of barrier or mound around the perimeter of the pond so that run-off has a harder time entering the pond. Of course this won't protect from run-off coming from a creek or stream, but can still help if you have the time and resources. Some folks also opt to treat the surrounding grass and soil near their pond with non-chemical treatments so that there is less fertilizer available to run-off into the water.

Each of these nutrient control measures should not be looked upon as a stand-alone solution. When possible they should be combined for a complete, holistic nutrient management approach.

Sunlight Reduction

There are fewer options for reducing sunlight into your pond, particularly if you have a large pond out in the center of a field or pasture, but there are a couple of effective ones. Probably the most common is to add a dye or colorant to the water. These products commonly tint the water blue and help with limiting the amount of light penetration the sunlight can get, thus helping to limit the algae's ability to photosynthesize. Of course these products, while effective, are also highly subject to personal preference, as many people prefer not to have blue colored water.

Another option for blocking sunlight is to add other aquatic plants to help shade the pond surface. Plants like water lilies or lotus can be planted and do a good job of shading the surface, while other floating plant species can also offer shade and will also absorb nutrients from the water. Adding aquatic plants to small ponds and water gardens are easier than adding to large ponds. Typically in a small pond, the plants can be left in their planter so they are easy to manage and control. Actually planting aquatic plants in a large earthen bottom pond can be a risky proposition as they can get out of hand quickly if not properly managed. A general rule of thumb would be a 50%-70% coverage rate.


Lastly, adding aeration to your pond could be the most significant and impactful tool for controlling algae. Oxygen is what makes a pond thrive and when it is deprived of it, it can suffer...big time.

Algae hates well oxygenated, circulating water and a proper pond aerator can provide both. There are different types of aerators for different types of ponds. The depth of the pond will usually tell you what type you need. Deeper ponds will normally require a bottom-based diffuser set-up while shallow ponds will typically use a floating aerator. See our article on Diffused vs. Surface Aerators for more info.

Thorough oxygenation will help break down organic waste more speedily by invigorating beneficial pond bacteria as mentioned above. Aeration also makes the use of algaecides safer as it protects against fish kills if too much algae dies off too fast. To a minor extent, aerators can even help with light penetration as surface aerators create such turbulence at the water's surface that sunlight cannot easily penetrate.


Effective algae control begins with knowledge and understanding of the type of algae you have and what causes the algae to grow. Once you have armed yourself with the proper information, treating the causes and controlling the sources become more manageable. Then, year after year, preventative maintenance becomes second nature and severe algae blooms become less and less of a concern.