Why use pond filters?
When to use pond filters
The choice of filter for your pond should be based on the size of the pond, the number and type of fish and the area available to accommodate the filter. Simple ponds with few or no fish should not require filters.
To work out the volume of water in your pond:
- Multiply the pond’s average length x average width x average depth ( in metres or feet to get cubic metres or feet)
- Multiply this figure by 1000 to get the volume in litres
- When calculating pond dimensions in feet, convert the cubic feet measurement to gallons by multiplying by 6.23.
Different types of filter are available. This information should help you choose the right product type for your purpose. If in doubt, speak to a specialist supplier before purchase.
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 (dead organic matter, uneaten fish food, fish excreta etc) 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. Suitable for small or large ponds, 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.
Filters can be divided into four categories:
- 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 will cause beneficial bacteria to die
- External pressurised filters
Water is pumped into the filter under pressure and returned to the pond under pressure. The filter can be installed discreetly at a position on the edge of the pond (and can be disguised with rocks or plants). Return water can be pumped up to a waterfall or used for a fountain jet. They are easy to maintain but should not be used on heavily stocked ponds or those with large koi carp
- External pump-fed filters
Water is pumped from the pond into the filter under pressure, but drains out by gravity. You cannot run a fountain using the outflow from the filter. The filter must be installed with the outlet above the pond water level. Pump-fed filters are easy to install and do not require any alteration to the structure of the pond
- 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 in the ground so that 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 and therefore can be used to power a fountain jet, a waterfall or a venturi if required
Ultra violet clarifier (UVC): Using an Ultra Violet Clarifier in your pond will ensure that the water stays clear of algae. UVC’s work by making the algae in your pond clump together so that it is large enough to be removed by your filter. Often, filter boxes will come with a built-in UVC but, if you are looking to add one to your existing system, then you will need a standalone UVC.
Additional factors affect filter systems. A major cause of failure (e.g. a continued algal problem) is when fish populations and feeding regimes are not taken into account. Water surface area, pond depth and exposure to sunlight also affect the size of filter required. Speak to specialist suppliers for advice;
Installation: always consult a qualified electrician before installing a filter system.
Maintenance: always follow the manufacturers recommendations regarding cleaning or replacing the filters (and replacing the UV filters where applicable).
The Royal Horticultural Society is the UK’s leading gardening charity. We aim to enrich everyone’s life through plants, and make the UK a greener and more beautiful place.