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Who doesn't want to look at a pond with fish swimming about peacefully in crystal clear water?

The desire to have clean pond water is the most common driver behind why people want to filter their pond, however, you need to understand what you are filtering and why.

We are just trying to finetune what would otherwise occur in a natural ecosystem.

The question is often asked "Do I need a pond filter system?"

Large ponds...

In a large well designed pond it should not be necessary. Lakes and dams through their volume and size have a much more stable environment. Just as with larger ponds, they can develop their own natural ecosystem, provided that there is the right balance of sunlight, water depth, water volume and plants.

A large pond is a much more stable environment and is less susceptible to changes. However, the extremes of temperature, light levels, high fish populations and/or low plant numbers can still affect the water balance. Thereby creating an environment where algae can flourish and cloud the water.

In instances such as these there may be simple remedies such as introducing more plants.

Smaller ponds...

In smaller ponds, however, the outside changes in the environment have a far greater effect on the pond environment. Strong sunlight quickly warms the water, whereas cold nights does the reverse. It is harder to maintain the ecological balance and especially if high fish numbers are to be maintained, then a pond filter system should be included.

Do I need plants and a water garden filter system to create a healthy fish pond?

Submerged plants, sometimes referred to as oxygenating plants, are great water clarifying plants, through the fact that they consume the same nutrients in the water as algae.

What would you rather have – green water or more plants? This balance is more difficult to achieve in smaller fish ponds, as small changes can have major influences in the pond environment.

The solution is to include a biological filtration system to help consume the nutrient load. Some even have UV lights that can kill algae.

The 3 types of water garden filter systems...

There are numerous water garden filter systems on the market, but they can be broken down into 3 catagories.

Mechanical filters

Mechanical filters that trap and remove sediment and debris. They may be skimmers that collect the leaves from the pond surface or various forms of mesh screens and sponges.

Biological filters

Biological filters provide a medium for beneficial bacteria to grow in and biologically detoxicfy the pond water. The bacteria feed on the toxic ammonia produced from fish waste and decomposed plant matter.

Ultraviolet (UV) filters

Ultraviolet (UV) filters are designed to sterilise the water passing through them.

How do the various water garden filter systems work?

Pond skimmers act just like swimming pool skimmers. They remove debris before it sinks to the bottom of the pond and reduce pond cleaning.

Mechanical filters remove larger debris and sediment from the pond water via a mechanical screen, mesh or sponge. The finer the screen the finer the particles they collect. Some small cheap pumps have small sponges as a filter. They regularly block up and reduce the pump flow, requiring more regular cleaning.

In simple terms, small filters mean more work. A mechanical pond filter system does not effectively filter out the green water algae.

Biological filters are the most effective way to detoxicify the pond water, by creating an environment that natural bacteria can grow in and thrive. To work best, the bacteria need to attach themselves onto surfaces that have water passing over them. The greater the surface area the more bacteria. The more bacteria, the better the biological filtration. These bacteria need oxygen to live, so they need the pond water to be continuously pumped through the filter 24 hours a day, 7 days a week. If the pump stops, due to a blockage or is turned off, the bacteria can use up the oxygen and die. The bacteria feed on the nutrients such as ammonia, nitrates and nitrites that build up in the pond from fish waste and even decomposing plant material. As a by product, the bacteria produce nitrates and nitrites which are then a food source for the plants.

Biological filters should not be washed or scrubbed clean as you end up destroying the bacteria colony, which takes about 6 weeks to re-establish. If there is an excess build up of silt and sludge, then a very light quick rinse in a bucket of pond water is all that is required, to allow the pond water to flow freely through the filter again.

Ultraviolet (UV) filters are sometimes referred to as UV clarifiers and UV sterilisers. In these filters the pond water passes through a tube that has a UV light bulb inside. The UV light bleaches and kills the green water algae.

It is often used in conjunction with other filtration methods as the water passing through must be relatively clear of particles in order for the light to shine through and so to operate more effectively. They are best combined with biological filters, which then trap the dead algae in the biological filter. The naturally occurring beneficial bacteria then feed on the decomposing algae, and help to clarify the pond water.

What is the best pond filter system?

The answer to this question varies according to the size of the pond and the desired outcome that you wish to achieve. As with any pond filter system, there are advantages and disadvantages, but all you are trying to achieve is a balanced ecosystem.

Large ponds can be designed so that there is little if any need for a water garden filter system. The trick here is to establish an ecological balance, often through good plant selection, so that healthy pond water can be maintained. The vegetation becomes the biological pond filter system.

Most domestic gardens do not have sufficient space available. As a result, any pond designed for a smaller space, while trying to replicate Mother Nature, may need a little help. In these circumstances we recommend a good biological filtration system combined with a mechanical filter such as a skimmer.

Just be aware that if you introduce fish into the pond you may need to increase the size of your biological pond filter system.

Small ponds, especially with fish, should also include an ultraviolet filter. The nutrient load caused by fish waste in a small pond is often difficult to manage with little room for lots of plants. There are many commercially available, small water garden filter systems, that incorporate an ultraviolet light combined with a biological filtration system.


Always seek expert advice when choosing or trying to size an appropriate water garden filter system.

A common mistake is to tell your supplier that you don’t intend to keep fish, then, after it is installed change your mind. The pond filter system may often need to be twice the size to cope.

What to avoid...

The best pond filter system is only as good as the pond design and pond maintenance will allow.

  • If you have drains or garden run off flowing into the pond each time it rains. You are introducing more dirt, nutrient and debris which make achieving a balanced pond ecosystem all the more difficult to achieve.
  • Leaves and excess dead organic matter should be removed to reduce the nutrient levels.
  • Over stocking with fish. Start with smaller sizes and numbers then as the pond establishes, you may introduce more. Remember that fish grow too.
  • Only feed fish with what they can eat in a few minutes once or twice a day. Excess food is wasteful and only feeds the algae. During the cooler months, their metabolism slows, feeding should then be reduced and often there is little if any need to feed them at all.

If you would like one of our Pond techicians to come out to help you,

please give us a call on (03) 9737 9663 or send us an email

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