Choosing a filtration system

Choosing a filtration system

Biological filters

Biological filters usually contain a series of sponges for mechanical filtration and biological medium of varied types for conversion of pond toxins. The biological filtration component may consist of bio-balls, pieces of plastic, rocks or any number of different things. The medium used should have a massive surface area for growth of healthy bacteria in the filter system. Biofilters can be pressurised or gravity style and many contain built in Ultra Violet (UV) Clarifiers for optimum water clarity.  Most small to average sized pond systems are set up using pressure filters as these can be buried in the ground beside the pond and can easily be added to an existing pond. Gravity filters have to sit at the highest point in the pond system, often looking unsightly in the garden unless provision has been made to hide the filter when designing the pond. Pressure filters may also have backwash and pump action functions for ease of cleaning

There is no problem using a pressure filter for a Koi pond (although there is much misinformation with regards to this). What is important is to select a filter of appropriate size for the pond and fish stock. Up until recently people with large ponds had no option but to run a gravity filter as large pressure filters did not exist. The most common reason given for using a gravity filter in a Koi pond over a pressure filter is that the gravity filter will provide increased oxygen exchange. However, there are many ways of increasing oxygen levels in the pond system, which are generally preferable to having a big filter box sitting beside your pond. For the most part it is mismatched equipment (pump inappropriately sized for the filter or filter too small for the pond) or user error (frequent over cleaning), which result in filtration systems being unsuccessful.
When choosing a biofilter for your pond consideration needs to be given to intending stock rate and the pond depth. Shallow ponds have greater filtration requirements than deep ponds as sunlight reaches further into the pond increasing water temperature resulting in increased growth of algae and potentially decreasing oxygen levels. For filtration purposes a shallow pond is generally considered to be anything less than 75cm deep, with a very shallow pond being anything less than 50cm deep.
As a general guide:
  • We recommend you disregard the model name of the filter and instead look at the recommended flow.
  • The rated or recommended maximum flow through the filter is the maximum size of pond in litres that the filter should be used for in Australian conditions. This is for average fish stock. If you plan on having large Koi then halve this number again ie if the filter has a maximum flow of 5000 litres then it is only suitable for cleaning a heavily stocked pond of 2500 litres.
  • Pass the water in your pond through your filter every hour for ponds with standard fish stock
  • Consider the amount of debris entering your pond system when choosing a filter. Ponds with large amounts of dust or high leaf litter entering the pond require greater filtration.
  • It is better to have a filter that is over capacity than under capacity, so when in doubt choose a bigger model.
Natural plant filtration

Natural plant filters utilise plants to clean the pond system. They require a separate pond or spill where water is forced to pass through plant roots, where nutrient is removed from the water as it flows. We use a massive plant filter to keep our pond system clean at Swan Valley Fish and Lily Farm. Many people use a combination of plant filtration and biological filtration and we believe this is the best possible solution for providing a healthy pond for your fish. Plant filters need to be sized appropriately for the pond in order for them to work effectively. As a general guide 1 square metre of plant filter will filter approximately 2000 litres of water. Heavily stocked and/or shallow ponds may require even larger plant filters. If you are unsure as to your filtration requirements, always make the filter larger if space allows. The plant filter itself should be packed with a mixture of bare rooted plants (no soil, pots or gravel). It is possible to utilise the plant filter to grow edible water plants such as Watercress, Lebanese Cress, Water Chestnuts and Kang Kung. It is important not to oversize your pump when using a plant filter, if water flow is too rapid, water may bypass many of the plants and filtration will not occur. For very large plant filters a series of channels may need to be constructed within the filter to ensure a long slow path of water through as many plants as possible. In addition to the plant filter we recommend the application of fine river gravel to a depth of 10cm across the base of your pond. This gravel bed assists filtration, providing a medium for healthy bacteria to grow for the conversion of pond toxins.
Still pond with no filtration

It is possible to have a still pond or water bowl with no filtration, these types of ponds are suitable as frog ponds and small goldfish ponds, they are not suitable as Koi ponds.  The key to having a healthy, clean still pond is a massive quantity of pond plants. Most of the surface area (approx 70%) of the pond or bowl should be covered with a variety of different plants. Fine river gravel or Zeolite gravel should cover the pond base, approximately 5-10cm deep depending on the depth of the pond. Fish stock should be kept low in a still pond. Ideally you are trying to create a natural eco system that looks after itself. It may take up to 12 months for a still pond to find it’s “natural balance”.  However, in nature things can and do go wrong, extremes of weather are more likely to create havoc in a still pond, this may cause a pond that has been running successfully for many years to suddenly become “unbalanced” resulting in algae blooms or fish death.