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How to Control Algae in a Large Pond

Is your pond 6 ft. deep or deeper? Do your fish seem to be gasping for oxygen at the surface of the pond? Is there a lot of algae in your pond? Is it lifeless and stagnate? If you can answer “yes” to any of these questions, please read on. You have insufficient oxygen levels in your pond, and you will struggle to control algae in your large pond.

Ponds that are deeper than 6 ft. are often unable to produce enough oxygen at those depths. This lack of oxygen will lead to thick nutrient layers on the pond bottom.  These nutrients will encourage more algae growth.  The human’s knee-jerk reaction is to add chemical algae killers, which simply exacerbates the problem more than we even realize.

Algae control is essential for maintaining a healthy pond. Algae blooms may discolor water, further reduce pond oxygen, and can asphyxiate other life forms if present in significant quantities. Excessive algae growth typically signifies water problems and can make the water unsuitable for swimming, fish, and other animals.

Table of Contents

Nutrient Pollution

Excessive nutrient pollution such as potassium, carbon, phosphorous, and nitrogen can originate from decomposing lawn clippings, fertilizer, septic systems, and farm runoff. For The most part, when you see a pond with an excessive algae bloom, it is the fault of the humans around the pond, not the pond.  The pond is simply reacting to the conditions we have subjected it to.

A pond’s health is all dependent on the ecosystem and the circle of life.  When we disrupt this balance, a pond’s condition can deteriorate rapidly.

Low Oxygen Levels

The bottom of the pond runs low on oxygen first. The decomposition of the debris on the pond bottom depends significantly on the oxygen levels at that depth.  The absence of oxygen kills the bottom-dwelling animals (worms, snails, mussels, etc.) and aerobic bacteria that help keep the pond clean. The loss of these tiny pond helpers will significantly reduce the pond’s ability to clean itself.

Nutrients such as dead algae, grass clippings, leaves, plant debris, and animal waste fall to the bottom of the pond continually, so if the process of decomposition becomes interrupted by lack of oxygen or chemical treatments, the pond water quality can deteriorate quickly.  This ever-thickening layer of nutrients will be the food for future algae blooms.

How to Increase Oxygen Levels

The best thing that we can do for a larger pond is to provide aeration. There are two types of aeration systems; bottom aeration and surface aeration.

Surface Aeration

Surface aeration is just like it sounds.  It is usually a floating electrical water pump that shoots water into the air.  This water shooting into the air picks up oxygen and falls back into the pond.  The action of this water flying up into the air and landing back in the pond will typically effectively circulate the top three or four feet of your pond.

Bottom Aeration

Bottom aeration is, again, just like it sounds.  We place an electric air pump near the pond, and we pump air down to the bottom of the pond using weighted air lines.  At the bottom of the pond, the air is pushed through perforated membranes of one type or another.  These membranes are called diffusers.  The diffusers create tiny little air bubbles that add oxygen to the bottom of the pond and circulate the entire body of water.  This is very similar to the bubbles coming up from the bottom of a fish tank—the same concept.

One bottom aerator will create a circular flow of water in the pond as the air bubbles rush to the surface carrying the colder deep water to the top, which causes the warmer topwater to flow toward the bottom.  The deeper the pond, the wider the water turnover area will be.  A shallow, long pond will require more diffusers.

This continual turning of the water column by pushing air to the bottom of the pond will help keep the oxygen levels high on the pond’s bottom.  ThisThis will help all of the bottom-dwelling animals and bacteria thrive, allowing them to be more efficient at decomposing and consuming our pond muck, which will help starve the algae of the nutrients it needs.

One note of caution would be that if you install a bottom aerator in a deep old pond full of muck, the aeration will turn over so much water and carry so much of the muck into the water column that it can kill fish and other pond animals.  When installing that new aerator, only run it for a few hours a day for the first week or so and then gradually increase the run time until you can safely run it 24/7.

Prevent Nutrients from Entering

Do everything that you can to eliminate runoff into your pond.  The less runoff that enters the pond, the less nutrients it will carry.  Of course, many ponds are strictly runoff ponds, so eliminating the runoff would be a real problem. 

To reduce the amount of nutrients entering a runoff pond, make sure to maintain a dense border of plant material around the outside of the pond.  The more plants that the runoff water runs through, the cleaner it will be when it reaches the pond.

If paved areas or a valley run into your pond, it is a good idea to provide plant-filled settling areas where sediment can settle out of the runoff and nutrients can be kept out of the pond.  Fast-moving water will carry many nutrients and heavy sediment loads, so it is a good idea to slow it down as much as possible.

Make sure to minimize any fertilizer sources, lawn clippings, leaves, twigs, road salt, or any other chemical that may enter your pond.  You can assume that everything uphill from your pond will end up in the pond, so use your best judgment and be careful.

Beneficial Bacteria

There are many different strains of beneficial bacteria that can be added to your water.  There are many liquid bacteria applications that are well suited to smaller ponds and weekly or monthly maintenance, but for larger, deeper ponds, once you get your aeration system working, you will speed the decomposition process by adding more decomposers (bacteria) to the bottom.  Look for sinking bacteria tablets such as Muck Buster or Muck off, which will sink to the bottom and give your decomposers a helping hand.

Whatever product you buy, make sure that it is entirely natural and chemical-free.  Just because a product may be labeled safe for fish does not mean that it is safe for your pond.  You only want completely chemical-free products.

With a natural bacteria product, you can never add too much, as the bacteria will be put to good work until the muck is so decomposed that there isn’t enough to support the bacteria levels. At this point, the bacteria will simply die off and be consumed by other water dwellers.


A straightforward and long-term solution to help keep your water clean is plants.  Lotus and lilies reduce direct sunlight and provide shade, which helps control algae’s growth. Submerged plants such as hornwort, parrot’s feather, and anacharis release oxygen into the water and compete for nutrients with the algae.  All marginal aquatic plants such as cattails, rushes, iris, reeds, etc., will compete for the same nutrients that the algae need to thrive.

A Problem With Plants

The more beautiful plants we have in our pond, the fewer algae we will have.  One trouble with this in large unlined ponds is that these marginal aquatic plants can really take hold and will often spread and overwhelm a pond in a number of years.

We need the plantings in our ponds to compete with the algae for the nutrients to keep the algae in check, but we don’t want them to overgrow the pond.  Often, this dilemma leads people to spray their pond with herbicides to kill off the plants.  This leads them into a vicious circle of chemical use and algae bloom, as every time they use chemicals, they are killing off beneficial pond life, and every dead clump of plant roots is now acting as fertilizer for the next algae bloom.  This is a never-ending, terrible situation.

A Few Solutions

So what can we do?  If you have pond plants overtaking your pond, the best solution might be to excavate or dredge them out.  This will remove the root mass and give you a fresh start.

Once you take care of that, you will be left with a pond that needs plants.  There are two ways to solve this problem that have proven very effective.

Floating Plant Islands

Floating plant islands can be purchased or built yourself.  The composition of the island isn’t as crucial as its function.  It must be constructed of a material that will float, supporting plant growth and letting the roots grow through into the water.  Most commercially available plant islands are made from recycled plastic and are a fibrous mat that is very buoyant and very porous.  This allows the island to support a significant amount of weight and allows the plant roots to hang down into the water.

These floating plant islands mimic the floating bogs found in nature.  The effectiveness lies in the fact that the plant’s roots are hanging into the water, soaking up nutrients.  The mat is just floatation; you don’t need to add soil because the entire point is to have the plants take nutrients from the water. 

The larger your pond is, the more coverage you will want.  These islands are ideal.  You get the benefits of plants in your pond without the disadvantage of having the plants spread throughout the pond.  The islands are usually roped to the shore in two places to keep them out in the pond or anchored to the pond’s bottom.

Foliage Removal

For the ultimate nutrient removal, you can cut the foliage from the plants at the end of the season and dispose of them so that the decaying plant material doesn’t add nutrients back into the pond.

Floating islands are increasing in popularity to improve water quality and provide an attractive focal point in the pond. Plants grow remarkably well on a floating island. The same vegetation planted on the shoreline near an island doesn’t grow nearly as well. Floating island plants grow bigger and healthier than plants along the shoreline.

Growing Wild

If you prefer to let your floating island grow wild, try a native garden. Depending on where you live, there may be local sources for plants and flowers. Your local garden center can help point you in the right direction. These don’t need to be all aquatic plants!  Depending on the type of island you install or build, you may be able to plant terrestrial plants that sit high enough to do just fine. Just don’t pick plants that like dry soil.  They need to like it a little wet. Choose plants that attract birds, bees, butterflies, and other species.

You can do whatever you’d like with these islands.  Get creative. As long as plants grow on them and roots hang into the water, they help control your algae and improve water quality.  You can plant the island to complement the existing landscape, grow a floating flower garden, zen garden, or create a specialty island of your design. 

No green thumb is required to grow an edible floating island. Grow any edible plant in your floating garden. Choose from edibles such as fresh herbs, fruits, vegetables, salad greens, or flowers. Planting it is simple and provides the perfect environment for natural and rapid plant growth with no pesticides or herbicides.

Floating Treatment Wetland or FTWs

FTW’s are very similar to standard floating islands but are typically larger and planted more diversely to allow for better habitat for other animals.  Are you looking for a natural way to cleanse your pond and a safe alternative habitat solution?  The FTW may be your solution.

FTW’s can be created to create habitat for waterfowl nesting, add shoreline protection, improve fish spawning, or greywater or stormwater treatment. There are many ways to make a beautiful and useful plant island. These are typically used in a larger, more commercial setting where greater stability and biodiversity are required.

The islands come in many shapes and sizes. The most common shapes are kidney, rectangular, or square. The islands can be joined together to make a larger floating island. The islands also come in circle shapes but are not as cost-effective as the other shapes.

The plants to use on your floating island will depend on the benefits you want to achieve from your island. Do you wish to beautify your pond, reduce algae, create a nesting island? Benefits such as nutrient removal, enhancement of habitat, the attraction of invertebrates, butterflies, or dragonflies can all be possible with one plant island, or you can create several different islands for differing effects.

Wetland Filter

Another great way to get all of the advantages of having plants in your pond without worry about them spreading is to create a wetland filter.  A constructed up-flow wetland filter is located outside of the pond.  A rubber-lined pit is lined with various-sized rocks.  Water is pumped from the pond into the bottom of the wetland filter, where it is forced up through the layers of rock.  The water then flows back to the pond via a waterfall or stream.

The wetland filter can be planted thick with aquatic plants.  The water getting pumped through the plant roots will allow the plants to absorb large amounts of nutrients to help control the algae in your pond.  Aquatic plants seem to absorb more nutrients and grow more quickly when in moving water, so the wetland filter creates super-charged aquatic plants.

Wetlands also help clean your water by helping to remove ammonia and heavy metals from your water and providing an excellent place for sediment to settle out of the water column.  Once again, to maximize water quality effects, the foliage can be harvested at the end of the season so that nutrients and removed toxins don’t re-enter the water.

 Understanding Algae and How to Control it in Your Large Pond

Many pond owners have a problem with algae but do not understand what they are and how they grow.

While most people don’t consider algae to be plants because they don’t have stems or leaves, they are very simple, primitive plants that use the process of photosynthesis, just like other plants, to take energy from the sun and combine water and carbon dioxide to form sugars for energy and growth. Algae also produce oxygen like other plants and absorb oxygen when there is no sunlight.

There are two common forms of pond algae.

  1. String algae, also known as hair algae, is a filamentous species that grow in long strands, adhering to rocks and waterfalls. They will eventually tangle together to form thick, unsightly mats that can double their weight within 24 hours.
  2. Green water algae are single-celled organisms that remain suspended in water. They are so tiny they can pass through even the finest filter. If there is plenty of sunlight and nutrients in the water, up to five million cells per milliliter of algae can be in the pond water.

Balance is the Answer

Balance is the best when it comes to controlling algae. No natural pond is ever algae-free, but it can be kept in check in a balanced environment. Understanding how algae grow is a good start.  Understanding that chemicals and sterilizing gadgets are never the answer is an even better start.

The realization that it is our job to simulate mother nature’s ways of creating balance and harmony in our man-made ponds is the step that many pond owners never grasp.  We should never fight with nature; we should step back, learn from nature and figure out how to allow nature to help us.

It sounds simple, yet it is so very hard for some.  Patience and understanding is the answer.  We often want a quick fix, so we grab a chemical, dump it in, kill the algae and think we are done.  We don’t realize the headache and never-ending battle that we have just started.  Using Mother Nature’s way through the addition of plants and bacteria coupled with some mechanical aeration, we can create a simple, long-lasting solution that will cost much less long term both financially and environmentally.

Don’t Be Fooled

Natural pond balance works for the control of algae. Don’t let anyone tell you otherwise.  Most pond companies will promote needless monthly chemical applications to maintain a pond. They are merely selling a product that will poison the water. If it is done the natural way, the pond will eventually do very well independently with no help from us.

When we use chemicals, it is a vicious cycle of killing algae with chemicals, and the dead algae go to the bottom, creating more nutrients for the next algae bloom when the chemicals wear off. This is what makes most natural bottomed retention ponds such a nightmare for their owners – chemicals and thick muck on the bottom of the pond.  Job security for the chemical application companies for sure!

Use natural means to control algae growth in your pond. The fish, insects, and plants will thank you – if they could.

Mimicking Nature

Using plant islands and wetland filters, we are mimicking the way that nature takes care of the natural ponds and lakes on this earth.  Working with mother nature is always easier than working against her.

A Pond is a Pond – Treat it Right

Whether you have a large backyard pond or are the manager of a retention pond or golf course pond, your hardships are often the same.  You want clean and clear water that is not thick with algae. If you have been trying to maintain this pond for a while, you are likely frustrated by the lack of simple fixes and the fact that most professionals will tell you that they need to apply chemicals weekly or monthly for eternity to control the algae.

I’m here to tell you that they are wrong, very wrong.  There are many simple and natural solutions to your pond troubles, and once you get your pond balanced and functioning as it should, your maintenance headaches will be eliminated.  Treat it right and it will do the rest.