The role of pond plants in maintaining a stable fish pond environment.

Virtually every environment on earth relies on plant life to produce the first food that fuels all the other living organisms in the ecosystem. That is certainly the case in a ‘natural’ mud pond, but not so in an artificially filtered koi pond that is plant-free by design. So how might this affect the quality of our koi’s environment and how does it really differ from a natural, planted alternative?

Throughout this series we have been taking a ‘behind-the-scenes’ look at the living processes that work together to create a healthy pond environment. Where it has been useful, we have compared a typical filtered koi pond with a more natural mud pond.

A mud pond embodies a natural balanced ecosystem that provides koi with an unrivalled, supportive environment that handles waste produced by koi and other pond organisms, reprocessing and transforming them into beneficial products.

On the other hand, a clear, filtered recirculating koi pond that is heavily stocked relies on artificial inputs of food and a means of removing the excessive levels of the resultant waste. Even though both scenarios set out to achieve the same result, a koi pond does not achieve the rich and balanced environment that koi experience when grown in a natural mud pond. One of the starkest and most fundamental differences between the natural and manmade systems is due to the role that plants play in creating and maintaining a balanced pond environment.

In a filtered koi pond, we strive to provide our koi with a plant-free existence, where our objective is to produce a pond of clean lines and low maintenance, where the only attraction in the pond is the koi themselves. By taking this approach, are we not denying our koi what mother nature had intended?

After all, plants are to be found at the start of every food chain (see box out) performing a plethora of roles, from purifying water to acting as a food source. How does a mud pond benefit from plant life, and how (if at all) is the environment in a filtered koi pond compromised by living such a plant-free existence?

Plant life in a pond can be very diverse, ranging from single-celled algae (greenwater) and more filamentous algae (blanketweed) through to more substantial plants such as submerged oxygenating plants and emergent marginal plants who prefer to just get their feet wet. Whatever their form, plants are the producers and builders of any environment.

They appear to perform a piece of biological magic by using sunlight and a handful of inorganic compounds to produce organic plant material that you and I know as food. In this way, plants act like living solar panels, trapping and utilising the sun’s energy to produce food and hence store that energy in a form that can be released and utilised by organisms further up the food chain.

Through the process of photosynthesis, plants use the sun’s energy to split water into hydrogen and oxygen and take in CO2 as their source of carbon to manufacture foods such as sugars, starches, oils and proteins.

Plants provide us with a great insight into how a pond’s ecosystem actually functions, as they are the middle-men in lots of important bioprocesses within a pond.


Plants absorb this inorganic source of carbon (which is a by-product of respiring organisms – including plants) and combine it with other inorganic compounds to produce organic products (food) that are stored.


Plants can absorb nitrogen in different forms, including ammonia (NH3/ NH4+) and nitrate (NO3). These two compounds are similar to CO2 in that they are waste products from animal metabolism. Plants combine the nitrogen with hydrogen (gained from water) and carbon and oxygen to form proteins.


We know from our experiences with unwanted nuisance algae that phosphorus (in the form of phosphates) will fuel plant growth. These compounds are important in a plant’s production of lipids.

Micronutrients: Plants also absorb and utilise numerous micronutrients, taking them from the water to form many other vital products, including vitamins.

So besides being regarded as producers and builders, plants too fulfil a much needed purifying role, taking the pond’s unwanted and accumulating ‘muck’ and turning it into ‘brass’ in the form of stored food.

But the good news story does not end there. An additional life-sustaining product is produced by all plant life on earth – oxygen.

In the daylight hours, plants photosynthesise to produce oxygen as a by-product which dissolves into the pond water. You can often see the evidence of the mass of fine bubbles that are released during photosynthesis by observing rafts of blanketweed rise to the pond surface in the day (buoyed up by oxygen bubbles that become trapped within the filaments), sinking to the bottom at night.

Primary consumers will feed on the plant growth within a pond, grazing and ingesting this freshly manufactured food and are in turn eaten by fish and other organisms further up the food chain. It is this vital recycling link between absorbing waste products and converting them to food that is missing in a koi pond. Given that there is no competition between plants in a koi pond, you can easily appreciate the field-day that opportunistic algae may have in such a situation, given the opportunity. UVcs have put a stop to green water proliferating which makes life all the more attractive for blanketweed. Put yourself in the position of blanketweed in a filtered koi pond. Clear, sun-lit water, an abundance of nutrients (nitrates, phosphates etc) and no shading or competition from other plants – Blanketweed heaven! Is it surprising that we find it difficult to control?

So whether intentionally or not, as nature abhors a vacuum, blanketweed is trying to fill the gap in a koi pond, that is filled by plants in a mud pond. We try to resist its growth as strongly as possible, maintaining the vacuum (and nature’s missing link) by suppressing blanketweed. But the battle is relentless and we are usually happy to settle for an uneasy truce.

So how does a koi pond differ for not having the plant life that nature intended?

Koi ponds are faced with a potential accumulation of by-products such as nitrates and phosphates, especially through the excessive levels of food that are offered, relative to the pond’s volume. As there are no plants to remove these and other compounds, we must intervene ourselves. This will involve partial water changes and installing other water purifying hardware such as protein skimmers.

Besides nuisance algae, there is no natural food production in a koi pond. Not only does this deny koi from feeding on a fresh vegetable diet (something that we can try to supplement with lettuce and other vegetable matter) but the pond will also be devoid of the intermediary organisms that would be found grazing on plant life, performing other water conditioning tasks. Consequently, the water quality does not benefit from the maturing effects of a diverse population of pond organisms such as insect larvae and other invertebrates.

Koi are not able to benefit from the colour enhancing side effects of ingesting primitive and highly digestible algae. Koi that are reared in a natural mud pond and consistently ingest algae, exhibiting vivid colouration. Just as we have to offer our koi an artificial diet, we also must offer them alternative sources of carotenoids – in the form of spirulina ( a dried blue green algae).

In summary, yet again we can identify that a filtered koi pond is a compromise between viewing koi in a dense population and providing them with the best environmental conditions. By opting for a plant-free, low maintenance system, we deny the pond of vital conditioning, recycling and purifying action that plants perform, having to step in and fill that gap ourselves with water changes and other make-do measures.

The only discovered food chain that does not start with plant life occurs where the sun never shines – at geothermal vents at the bottom of the sea. As these vents emit mineral-laden water, chemotrophic bacteria process it and thrive on an abundance of ‘chemical energy’. Filter feeding and scavenging organisms feed on these bacteria, starting the food chain without either the sun’s or plant’s input.

Did you know?

Did you know that plants are green because they reflect green light? Chlorophyll is the photosynthetic pigment, absorbing blue and red light, making plants green.

Kill blanketweed and string algae.