Pond filters are used to maintain high water quality by filtering out both chemical pollutants and solid waste that can be potentially harmful to fish.

Different pond environments and different fish stock will create differing filtration needs. An ornamental goldfish pond with good planting will be a largely self contained environment with little or no need for filtration. At the other end of the spectrum, a well stocked koi pond, particularly one with minimal plant stock, will require a biological filter.

The biological filter uses bacteria that break down harmful ammonia produced by the fish themselves firstly into nitrites and then a further set of bacteria break down the nitrites into nitrates. Nitrates are compounds that support plant life within the pond environment and are beneficial providing they themselves do not become present in too large a volume.

The need for pond filters fundamentally stems from the desire to create a pond environment with considerably more fish stock than would exist in a similar volume of water in nature. The specific filtering needs of your pond therefore depend on how you intend to stock it. It is generally prudent to install a pond filter with a higher capacity than you may currently expect to need. This will give you an element of future proofing should you decide on further stocking of your pond at a later date.

Choosing a pond filter with a barely sufficient capacity may contribute to health issues and will certainly restrict the opportunity for increasing fish stock without either health risks or the need for further investment.

Once installed, the biological nature of the filter means there is a requirement to allow several weeks for the biological environment of the filter to become fully developed and the pond environment to become stable.

The filters themselves come in different forms with different characteristics such as gravel and crushed clinker amongst others. The filtration system works in conjunction with a pond pump system to drive the water through. The selection of the pump itself from a large range of devices and arrangements also requires careful consideration to ensure that it will meet the requirements of both the current and future pond environment.