Do You Need Pond Pumps and Pond Filters to Control Algae?

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Robert Pavlis

About 7 years ago I started developing various water features, both garden ponds and water falls. As part of my research I found consistent comments like the following:  you can’t make a natural pond using a pond liner without pumps and filters. The use of the word ‘natural’ here refers to the pond filtration system, not the esthetic look of the pond. I’ll deal with esthetics in a future post on how to build ponds.

In a natural pond the water, soil, plants, and animals all live in harmony. No one comes along to clean the pond or to aerate it. There is no big man-made filtration system that keeps the water clean. The common advice is that a pond liner is artificial and a pond built with it will never reach a natural state where the water, plants, and animals live in harmony the way they do in a natural pond. If you don’t filter such an unnatural pond it will become full of algae and the water will be dirty and smelly. The only way to have a pond with a liner is to add aeration and filtration.

Is this really true? Do you need pumps and filters to provide artificial pond filtration?

pond pumps and pond filters
Author’s pond at Aspen Grove Gardens (1 st year)

Building Natural Ponds

This blog post is the second most popular post ever on this site. Lots of people comment and are interested in more information about building natural ponds, so I have started a public Facebook Group to make it easier for people to discuss this hot topic. Please join the group at

Building natural ponds face book group
Building natural ponds facebook group

What Happens in a Natural Pond?

In a natural pond animals (insects, fish, etc) eat, poop, sleep and die. Both the poop and dead animals add nutrients to the water. Some is added immediately, and some is added over time as the material is degraded by various micro-organisms.

Food Science for Gardeners, by Robert Pavlis

Plants also add nutrients when they die. In fall all kinds of leaves and other dead plant material is blown into the pond, and as this material is decomposed by micro-organisms it also increases the nutrient level.

Algae is a plant that grows best with high light and high nutrient levels. When the nutrients get high enough, the algae takes over the pond and chokes everything else out.

Why does the algae not take over natural ponds? The answer is higher order pond plants (not including algae). Plants also use nutrients and as long as the plants in the pond use up the nutrients as fast as they are produced, algae has trouble getting a foot hold.

The secret to an algae fee pond is to control nutrient levels!

The other important part of a natural pond is the presence of micro-organisms. They are everywhere; in the soil, on rocks, and attached to plants. Think of these micro-organisms as the ‘cleaning machine’ of the pond. They take rotting, smelly animal and plant material and turn it into nutrients that plants and algae can use. The micro-organisms keep the water clean, and keep it from smelling.

A man-made pond made with a pond liner has no soil – so one source of micro-organisms is missing, especially if you keep cleaning the pond liner. Most ponds have few stones and few plants reducing the number of micro-organisms even further. Without microbes or filtration, the dead animals and plants just sit in the bottom, making the water cloudy and smelly. But it does not have to be this way – read on.

Controlling Nutrient Levels

There are a number of ways to control nutrient levels:

1) Don’t add too many fish. Too many fish results in too much fish poop. Koi poop more than gold fish.

Soil Science for Gardeners book by Robert Pavlis

2) Don’t feed fish. There are lots of natural things for the fish to eat. Adding extra food just adds more nutrients to the pond.

3) Have lots of living plants in the pond. With enough pond plants growing, they will remove the nutrients before the algae can grow.

Most man-made ponds are not designed to hold a lot of pond plants. Without the plants you need to add some type of mechanical filtration system.

how to build a pond or water features
Adding the pond liner to a water feature at Aspen Grove Gardens

The picture shows a pond at Aspen Grove Gardens during installation. The black pond liner is in the deep part and the planting shelves are covered in carpet (white/gray areas). The liner will be pulled up to cover the carpet.

Note the extensive size of the planting shelves compared to the total size of the pond.

Why Aerate a Water Feature?

A natural pond has no obvious aeration. There is no hidden pump creating air bubbles. So why is it needed in a man-made pond?

A poorly designed man-made pond does not have enough plants, and it does not have enough places for micro-organisms to live. As a result, dead stuff accumulates on the bottom. As this dead stuff starts to rot it uses up oxygen, and the water at the bottom becomes depleted in oxygen, which in turn causes anaerobic bacterial to grow. This type of bacterial loves the smelly mess and they thrive with low levels of oxygen. This seems like a good thing, and environmentally it is a good thing. The problem for us is that as they digest the rotting stuff, they make the water smell. We don’t like smelly ponds even if they are natural.

How do you get rid of the anaerobic bacteria? Simple, increase the level of oxygen by pumping air into the water.

Aeration is recommended for ponds so that they don’t smell and that works quite well. The problem is that without anaerobic bacteria, the sludge on the bottom degrades very slowly and so you also need to clean the bottom of the pond on a regular basis.

Think about this. Because you bought a pump and you aerate the water, you now need to do more work and clean the bottom.

Do Natural Ponds have Smelly Sludge at the Bottom?

Sure they do. It is quite normal to find this in a pond where a lot of animal or plant material falls into the water. You don’t normally smell it because the water is not stirred up enough to move the smells to the surface. Dig around with a shovel or step into it from a canoe and you’ll find the smell. The smelly sludge on the bottom is natural.

My man-made water features have sludge and anaerobic bacteria in the bottom. If I don’t disturb them, they degrade dead plant material, and produce nutrients for growing plants. In my ponds I don’t stir up the water so I don’t smell them.

natural ponds promotional ad

Pond Filters

Virtually every reference on building water features recommends some type of filtration system for a man-made pond. Why is this necessary?

As mentioned above, the lack of soil in the bottom of the pond and the lack of rocks and plants results in an environment that houses few micro-organisms when compared to a natural pond. The solution is to provide a man-made place for the microbes to live. Most filtration systems contain some type of surface for the micro-organisms to live on. This can be sand, wool, small pieces of plastic – it doesn’t really mater. What is important is that there is a lot of surface area. Microbes like to attach themselves to a surface and then ‘eat’ plant and animal bits as they float by in the water. So the filter replaces the natural places were microbes live–on soil, rocks and plants.

In nature the microbes live in the slime you find on rocks. It is healthy for a pond.

Water Feature Myth

The above information should give you some background to understand both natural and man-made ponds. If you think about it for a while you will realize that a properly designed pond, with enough pond plants and homes for microbes,  should work just as well as a natural pond. When I was planning to build my ponds I spent a lot of time trying to find a reference for a man-made pond that worked without pumps and filters – I found none. Every reference I found said that such a pond will not work.

I set out to prove the experts wrong.

The following is not the result of good research or the opinions of experts. It is the result of my experience with two ponds over a 6 year period. In this blog I am not describing how to make a pond but I will do that in another blog entry–some day. I will provide the key points to consider.

Based on the information above, a pond design needs two things which are lacking in most designs. It needs lots of pond plants. Plants will use up the excess nutrients and keep the algae in control. It also needs lots of little homes for microorganism – they will help keep the water clean.

The following are some key design decisions:

1) Wide planting shelves. My pond is 20 x 30 feet and about half of that area is in the form of planting shelves that are about 8 inches deep.

2) Soil is not used for the plants. Pond plants don’t need a substrate, except to hold them down, and the pond certainly does not need more nutrients from soil. The plants should be using the nutrients produced by the micro-organisms and not the nutrients in soil . Instead of soil, I use small rocks – 1/2 inch or so. I just use all the small rocks I collect as I make new flower beds. Don’t fertilize your plants.

3) Pond plants are not in pots. I just place them on the small rocks in the planting shelf. A larger rock on top holds them in place until they root.

4) The sides of the pond are lined with rocks adding more surface area for microbes.

5) A deep planting shelf (about 2 ft deep)  is also present for waterlilies. It is important to cover 2/3 of the surface water to reduce light getting to algae.

6) Goldfish were added to feed the plants, and eat mosquito larvae . They are never fed. They grow quickly and breed regularly.

The garden pond in the pictures was built 5 years ago and it is only now that the planting shelves are starting to be full of plants. Wild bull rushes seeded themselves the second year. Irises have been added and are spreading. The pond has never been drained, and the bottom is never cleaned. It has no pump and no filter. Wind is the only thing that might provide some aeration as it ripples across the surface of the water. There are several large trees around the pond that add fall leaf drop – which is left to settle in the pond.

For the first 4 years the pond plants increased in number each year. During this time, I did have string algae, but it mostly had a spring bloom and by mid summer it was under control. The water was very green showing the presence of lots of one celled algae, but the water was clean, and it did not smell. The fish that were added the first year did not over winter but since year two they have overwintered and keep breeding. Herons and racoons help to keep numbers in check.

From a naturists point of view the pond is very healthy with lots of frogs and dragon flies breeding each year. Larger mammals, including deer use it as a water source.

It is now nearing the end of summer 2013 and the pond has been extremely clean–much clearer than the picture below from 2012. In fact it is too clean. You can now see the pond liner in the deeper sections of the pond. There was no string algae this year and almost no one celled algae. It has been a strange year weather wise which may account for some of this, but I think it is mostly due to the fact that the planting shelves are now very full of hungry plants which are out-competing the algae.

After 5 years I conclude that aeration and filtration are ‘probably’ not required. I’ll need to wait another 5 years or so to be absolutely certain of this. It is possible that in a few more years the stuff at the bottom will overwhelm the pond and may need to be removed. I doubt it!

Water feature without pond filters or pond pumps
Water feature without a pond filter or pond pump 2012

String algae is gone, but one celled algae is still making the water green in late summer. Note the number of plants in the water.

Water Features at Aspen Grove Gardens
Same water feature as above in fall of 2014

As plants grow and get larger, algae is almost non-existant.

Is Green Water Bad?

From an environmental point of view there is nothing wrong with water that contains algae. In a natural pond it might indicate that too much fertilizer has leached into the pond which is not good. But this is not usually a problem in a man-made garden pond. If your pond water is green with algae it is probably healthy.

You might not like the look – that is a different matter.

In Japan, garden ponds are treasured and it is common to buy a dye to color the pond water. Why do they do that? When the water is colored it reflects light much better. The shadows and reflections are considered to be very desirable. So next time someone comments about your green water, just tell them that you do it on purpose to better enjoy the reflections.

If You Have Questions

If you have further questions about building natural ponds or about your existing pond please post them in our new face book group called Building Natural Ponds.

Beneficial Pond Bacteria

natural ponds do not need to have bacteria added. For more on this topic have a look at Beneficial Pond Bacteria – A Waste of Money.

Related Posts

Water Lilies for Ponds

Winterizing Ponds and Water Features

Selecting the Best Pond Liner

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Robert Pavlis

I have been gardening my whole life and have a science background. Besides writing and speaking about gardening, I own and operate a 6 acre private garden called Aspen Grove Gardens which now has over 3,000 perennials, grasses, shrubs and trees. Yes--I am a plantaholic!

283 thoughts on “Do You Need Pond Pumps and Pond Filters to Control Algae?”

  1. Hi Robert, great article.
    So you don’t use any form of drain in your liner either?
    And how do you prevent the fish from multiplying to much ? Or is that not a problem ?


  2. Hello. My brother and I built a natural pond in our UK garden when we kids, 1973/4 – dug a hole, put a liner in then lots of plants, then fish. We just didn’t know we were meant to have pumps etc! We kept tropical fish indoors so had some understanding of the nutrient / chemical cycle. It was very successful – clear water, gold fish bred and golden orme reached over a foot in length. I’m now over 60 and hoping to put in a pond in my new house’s garden. I would like a small stream/cascade to flow into pond – is this possible via a siphon effect? I’ve just bought your book and hope it will cover this. Thank you.

  3. Ponds longer than 6 feet on a side and deeper than 18 inches require so much digging and other heavy work that they are best left to pros. Smaller ponds are good DIY projects, but let pros handle the plumbing and electrical work.

    • Why? You can rent an excavator and do it yourself? My smaller pond is at least 6 ft wide and 3 feet deep – I dug it with a shovel.

    • A year ago I dug by hand a 15′ x 20′ x 3′ deep pond, with lots of limestone that had to be split apart to remove. I was 70 years old and enjoyed the work, using my garden tractor and small trailer to haul away the dirt. We used much of the rock around the perimeter. I have a waterfall with circulating pump but no filter, and with lots of vegetation we have very little string algae. Goldfish and shebunkins, no koi and of course we don’t feed them. Frogs showed up on their own. Until our vegetation multiplied we ran monofilament across the top which kept away the herons.

  4. Hello
    I inherited a small plastic pond in my garden in North England with no filters, pumps, fountains or falls. The original goldfish have turned black and are now rarely seen. Waterlillies cover most of the surface in the summer with lots of insects and dragon flies. This year there was a lot of blanket weed in the spring but when I fished it out with a net there were newts with every catch. There are frogs as well and occasionally a grey heron visits. Wonderful!

    • I live in Australia and have also experienced the gold fish going black. When I set up an indoor goldfish tank I caught a couple of my own breed black goldfish. After a couple of weeks in the clean filtered water they were gold and red colored same as the originals had been.

  5. I am really happy to say it’s an interesting post. I learn new information from your article, you are doing a great job.

  6. I can’t wait for your book to be released! My hubby & I just bought a farm in western Oklahoma with an already established large “pond”. It has a water fall – pump and filter system. I don’t want to use chemicals as the deer, raccoon, squirrels, lizards, frogs, etc live and drink out of it. Like yours, I’d love to have it take care of itself. I’ve had a hard time figuring out how to make this non-chemical idea work for me though. I love your research and ideas and would like to give it a try. I’d love your input on where to start….please. Right now, it has no fish, its full of green algae and dirty water. We covered it in the fall to keep more leaves out of it. You can see it in this picture:

  7. Great article. I built four ponds in my sloping backyard two years ago. The ponds flow from one into the next with the top pond having a 3′ water fall, the next two have about a 2′ waterfall each.The ponds range from about 300 to 500 gallons in size. I placed plants in the area around the ponds but plan to add water plants and fish this spring. I plan to put two KOI in the top pond, about 12 small goldfish in the second and the mosquito larvae eating minnow fish in the bottom two. I live in the U.S. in Iowa and will need to make sure my fish and plants survive the winters. The top pond is 4′ at the deep end next to the waterfall and slowly slopes up over a 20′ distance to the shallow end. I plan to build a barrier to keep the KOI in the deep end and keep them from eating my plants in the shallow end. The second pond is about 2′ deep or a little greater and is elongated somewhat like a creek. The bottom two ponds average about 3′ deep each. I have a 10,500 GPH pond pump which draws from the bottom pond and dumps into the shallow end of the top pond with a 6′-7′ rise in elevation. The pump and pipes are completely hidden, not visible at all. The pump runs silently and provides great circulation through the ponds and impressive water falls. The waterfall sounds are really impressive. I have a 2,000 SF house, 3/4 of the roof rain water flows off the back of my house. I routed the gutters to drain, one into the top pond and the other into the second. I never have to top off the water which is nice but I have had a very bad algae problem. I built every aspect of the pond myself and have been struggling with what filtration system to use. I started studying creating a pond system that does not need filtration and your article closed the loop for me. I have water plants on the way here, Jan 2017. I will start them in the house under grow lights then move them to the ponds in the spring. I will then add fish and I’m very excited about the results that I am confident in having. Thank you again for this wonderful article. My e-mail address is if anyone would like to contact me.

      • I will join, by the way I actually made an attempt to calculate the gallons per pond and it appears my largest one is much more like 1,200 to 1,400 gallons. The others are not too much smaller. Larger capacity than I thought, I wasn’t concerned in the least how many gallons they each had, I was going for what we wanted. When I started studying KOI, gallon size I found out is important for several reasons. Like best population size based on gallons of water and water depth available to them. It’s been enjoyable researching pond fish, their needs and desired water conditions. I’m looking forward to planting water plants and adding pond fish this spring. My grandsons are going to be involved with the process as well. Should be very enjoyable.

      • Hello. I live in an area in SoCal with a lot of mountains so it gets really cold and then really hot (some weather background). And the house I just moved into has an empty man-made pond that is completely lined with big rocks that’s about 5′-6′ long and about 3′-4′ at its widest. So it rained and it completely filled up so it’s about 3′ deep. Mosquito larvae have infested it so I just got 20 minnows and 20 goldfish to eat them. The puddle/pond has been here a while and the rocks already had moss. And I added 2 water plants. And I have orange trees around it where leaves and even some oranges have fallen and there’s brush around it too.
        What should I expect from my fish?
        Are the gonna die quickly or do they have a chance even if there is no water pump?
        Is the water too shallow?
        Also wondering will they freeze? It gets about 40 degrees at night over here sometimes. Thank you.

        • The fish will be fine – why would they die? The minnows may die in summer if it gets too hot – they like cooler water.
          Three feet is lots of water for small fish.
          Add a lot more plants – they need to use up the nutrients in the water. If the pond has not be maintained for a while, you might want to scoop out most of the stuff on the bottom.
          Assuming you are talking fahrenheit degrees, 40 is above freezing – so they won’t freeze.

          You might be interested in joining our Facebook group

  8. Thank you for this site as it’s inspired me to create a pond with my daughter this spring. I live in Maryland and am going to make a 7×12 pond with the intent of making one end three feet deep and the other down to 24-30″. I plan to have two shelves on the deep side and one wider shelf on the other. What do you think would be the best width of each shelf for this design? I do plan to have a a few goldfish as well as some mosquito fish and some bluegill to keep them in check. Any recommendations on the best layout for this size would be greatly appreciated!

  9. We have 3 ducks. My husband is very interested in a natural man made pond with a liner but he keeps saying the ducks will eat all the plants and quickly. Do you have any advice or suggestions on plants that ducks are less likely to eat?

    • Sorry I am not a duck expert. Suggest that you pose this question on the Facebook Group called ‘Building Natural Ponds’.

  10. Very helpful and informative article. I have been thinking of homesteading for years and when I do I would like to have one or two ponds on my land. I never considered putting in a pond with no liners or filters but you have proven that it could be done.


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