There are two main methods of filtration:

• Mechanical filtration – the purpose of this filter is to sieve out dirt, solid waste and algae. Water is drawn through foam, coarse sand, gravel and/or filter granules by the action of a submersible pump. This filter is inexpensive and usually installed in the pond. It is effective as soon as the system is switched on and can be run intermittently

• Biological filtration – waste products (such as uneaten fish food, fish excreta, other organic debris) and ammonia gas (which is toxic to fish) are all turned into harmless materials by the bacteria and tiny organisms that flourish on the filter medium. The bacterial population takes six weeks to build up and can die if the filter is switched off for 24 hours or more. It is suitable for small or large ponds and the unit is housed outside the pond, usually by the top of a waterfall. A surface or submersible pump is used to push the water through one or more layers of filter medium

The choice of filter for your pond should be based on the volume of the pond (see below for how to calculate this), the number and type of fish, and the area available to accommodate the filter.

If an electricity supply is needed to power the filtration system, this must be installed by a qualified electrician.

There are four main types of filter to choose from:

1. Internal filters – these are basic filters that are positioned in the pond. They are usually only used in smaller ponds that are lightly stocked with fish. These units incorporate a pump, filter and UVC (ultra violet clarifier) in one easy-to-install unit. The drawback of this type of filter is that water circulation is uneven, which may cause beneficial bacteria to die

2. External pressurised filters – water is pumped into the filter under pressure and returned to the pond under pressure. The filter can be installed discreetly on the edge of the pond (and can be disguised with rocks or plants). The returning water can be pumped up to a waterfall or used for a fountain jet. They are easy to maintain but shouldn’t be used for heavily stocked ponds or those with large koi carp

3. External pump-fed filters – water is pumped from the pond into the filter under pressure, but drains out by gravity. You can’t run a fountain using the outflow from this filter. The filter must be installed with the outlet above the pond water level. Pump-fed filters are easy to install and don’t require any alteration to the structure of the pond

4. External gravity-fed filters – these are usually only used if the pond has been specially constructed with a sloping base and bottom drains. The filter must be sunk into the ground so the water level in the filter is the same as the pond. Water enters the filter by gravity from the pond and is returned by the pump under pressure, so can be used to power a fountain jet, a waterfall or a venturi (for aeration) if required

Ultra violet clarifier (UVC)
These help to keep pond water clear of algae. They work by making the algae clump together so they’re large enough to be removed by your filter. Many filter boxes come with a UVC, but if not, then standalone UVCs are also available.

Pond algae and blanket weed

Pond algae and blanket weed

Before choosing a filter, you need to know the amount of water in your pond. This can be calculated in various ways, depending on which volume measurement you want:

• Cubic metres/feet: multiply the pond’s average length x average width x average depth (in metres or feet to get cubic metres or feet)
• Litres: multiply the cubic metres by 1,000 to get the volume in litres
• Gallons: convert the cubic feet measurement to gallons by multiplying by 6.23

To work effectively, pond filters need regular maintenance – cleaning or replacing the filters, and replacing UV filters where applicable. Always follow the manufacturer’s recommendations.

If you continue to have problems with algae after installing your chosen filtration system, then it may not be suitable for the number of fish and/or feeding regime. The water surface area, pond depth and exposure to sunlight also affect the size of filter required. Speak to a specialist supplier for advice.