A view below moving water surface,

Choosing pond filtration system – to filter or not to filter?

Filterless or with a filter? No matter which filtration system you choose, the aim of both is to keep your waters balanced. “Balanced” can also have varying meanings, depending on your pond characteristics and what are your expectations.

Let’s first look into the different filtration systems.

Natural “filtration” process (a wildlife pond) – the no-filter system

Nature has its own ways to keep the water clean and clear. A wildlife pond, designed to mimic natural ecosystems, relies on a process called natural filtration. This process is based on the hard work of beneficial bacteria and aquatic plants to maintain water quality.

The heart of natural filtration lies in the nitrogen cycle. Fish and other aquatic animals produce waste in the form of ammonia. Beneficial bacteria, known as nitrifying bacteria (nitrosomonas and nitrobacter), convert this toxic ammonia into also toxic nitrites and then into nitrates. These nitrates, though less harmful, can still pose a threat in high concentrations. This is where aquatic plants step in, absorbing nitrites and using them for their growth. In this article you can read more about this process.

Benefits and Limitations of Natural “Filtration” systems:


  • Nutrient Control: Aquatic plants consume nitrates as a source of nutrients, limiting their concentration and preventing excessive algae growth.
  • Oxygenation: Plants release oxygen during photosynthesis, ensuring an oxygen-rich environment for fish and other pond inhabitants.
  • Wildlife Habitat: Because of the presence of numerous plants, natural filtration creates a biodiverse ecosystem, providing food, shelter, and breeding grounds for a variety of organisms.
  • Cost-effective: Natural systems can be less expensive to set up and maintain compared to filtered systems.
  • Low energy consumption: Natural systems do not require electricity to operate, resulting in lower energy costs.
  • Environmentally friendly: These systems do not rely on chemical treatments, making them more environmentally sustainable.


  • Limited Effectiveness: Natural system may not be sufficient for ponds with high fish populations or heavy organic loads.
  • Limited control: Natural systems may not offer the same level of control over water quality as filtered systems.
  • Seasonal Variations: The effectiveness of natural filtration can vary throughout the year, especially during colder months when plants go dormant.
  • Maintenance: While natural filtration reduces the need for mechanical filtration, periodic maintenance tasks such as removing excess plant growth and debris are still necessary.
  • Slower results: It may take longer for natural filtration systems to establish and show noticeable improvements in water quality.
  • Requires a larger space: Natural systems often require a larger pond area to accommodate the necessary plants and organisms.
  • Limited removal of fine particles: While natural filtration can address larger debris and organic matter, it may not effectively remove fine particles from the water.

How to implement natural filtration in my pond?

Selecting Appropriate Aquatic Plants:

  • Floating Plants: Plants like Water Hyacinth and Water Lettuce float on the water surface, shading it and limiting sunlight, which inhibits algae growth.
  • Submerged Plants: Examples include Hornwort and Anacharis, which absorb excess nutrients and provide hiding places for small organisms. Here you can read about some of the aquatic plants.
  • Marginal Plants: Iris, Cattails, and Rushes planted along the pond’s edge help filter runoff, stabilize soil, and offer habitats for amphibians and insects.

Creating a Balanced Ecosystem:

  • Fish Selection: Opt for fish species that are relatively robust and compatible with a natural filtration system, such as goldfish or koi.
  • Beneficial Bacteria: Introduce beneficial bacteria supplements to jumpstart the nitrogen cycle and enhance natural filtration processes.
  • Avoid Chemicals: Minimize or eliminate the use of chemicals in your pond, as they can disrupt the delicate balance of the ecosystem and harm beneficial organisms.

By combining appropriate aquatic plants and maintaining a balanced fish population, you can achieve a thriving and self-sustaining wildlife pond. Remember, it’s essential to monitor water quality regularly and make adjustments as needed to ensure the continued success of your natural filtration system. 

Mechanical and Biological Filtration

When using a filter in a pond, there are two primary types of filtration involved in “cleaning” the water – mechanical and biological filtration.

  • Mechanical filtration involves physically removing debris and particles from the water. This is typically achieved through the use of mechanical filters that trap suspended solids, leaves, and other visible contaminants. Mechanical filters often consist of foam pads, filter mats or filter brushes that act as barriers, preventing larger particles from passing through.
  • Biological filtration relies on beneficial bacteria to break down harmful substances in the water. These bacteria convert ammonia, a byproduct of fish waste and decaying organic matter, into less harmful compounds. Biological filtration occurs within biological filters, also known as biofilters, which provide a large surface area for bacterial colonization. Examples of biological filters include trickle towers, bead or chips filters,  and moving bed filters. They are usually positioned after the mechanical filter in the filtration system to allow the water to flow through the bio-media.

In most of the pond filtering systems, both of those filtration types are used. Water goes first through the mechanical filtration, to get rid of the debris and then passes through a biological filter, where bacteria take care of the ammonia and nitrite break down. With time the nitrifying bacteria colonize the whole pond, so removal of toxic ammonia and nitrite happens constantly in the whole system.

You may be also interested to see how to choose a filter for your pond, regarding the size and needs of your pond.

Pros and Cons of filtered systems:


  1. Effective water purification: filtered systems can provide efficient removal of debris, pollutants, and harmful substances from the water.
  2. Faster results: These systems can quickly improve water quality and clarity, leading to a healthier pond environment.
  3. Greater control: filtered systems offer more control over water parameters and allow for targeted treatment if issues arise.
  4. Versatility: Different types of filters and filter media can be utilized to customize the filtration system according to specific needs.


  1. Higher cost: filtered systems can be more expensive to purchase and maintain due to the equipment and energy requirements.
  2. Energy consumption: These systems typically require electricity to operate, resulting in higher energy costs.
  3. Potential for over-treatment: If not properly monitored, filtration can lead to excessive removal of beneficial substances, disrupting the balance of the ecosystem.
  4. Maintenance requirements: filtered systems need regular cleaning and maintenance to ensure optimal performance.

The table below summarizes the pros and cons of natural and filtered systems:

Natural SystemFiltered Systems
ProsCost-effectiveEffective water purification
Low energyFaster results
consumptionGreater control
Supports biodiversityVersatility
Environmentally friendly
ConsLimited controlHigher cost
Slower resultsEnergy consumption
Requires larger spacePotential for over-treatment
Limited removal of fine particlesMaintenance requirements

Consider specific needs of your pond, your preferences, and resources when deciding between natural and filtered systems. You may also opt for a combination of both approaches, utilizing natural filtration with the addition of a mechanical or biological filter to achieve optimal water quality in your pond.

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