Pond Filters

Decomposition of fish waste and plant debris releases toxins into ponds and depletes oxygen levels in the water. Simple ponds with few or no fish should not require filters. However, they are essential for stocked ponds where imbalances in the pond's ecosystem are more likely to occur.

 

Biological filters

Biological filtration relies on naturally occurring bacteria that colonise a specially designed filter medium with a large surface area. The bacteria convert ammonia, which is toxic to fish, and other nitrogen-based compounds into nitrates. These are then utilised by pond plants, thus removing nitrogen from the water. Biological systems are essential for ponds with substantial fish populations (especially koi, which produce 40% more waste than goldfish). Unless bacterial supplements are used, the bacterial population takes six weeks to build up. If the filter is switched off for more than 24 hours the bacteria die and a further six weeks is required to rebuild the population. Biological filters only require cleaning at the end of the season. If cleaning is essential during summer, use pond water, as the chlorine in tap water kills bacteria.

Mechanical filters

Mechanical filters simply remove solids from the pond by straining water through gravel, brushes or foam filters. They can be fitted directly onto a submerged pump and are suitable for small ponds with only a moderate fish population. Regular cleaning is normally required to keep them working properly (about once a week in the height of summer) though self-cleaning models are also available. The filter is effective immediately it is switched on, unlike the biological types described below.

Combined filters

Combined filters that use mechanical, biological and even ultraviolet (UV) means are more suitable for larger ponds with large populations of fish. Most of these combined filters are pump-fed and have a gravity outlet, and so are positioned higher than the water return point. More complex pressure-fed models can be part-buried. Combined filters can be placed after specially designed water pumps which can handle solids.

Ultraviolet lamps

Ultraviolet clarifiers are placed at the front of the biological filter system. They act by killing green algae and make it flocculate (clump together), easing mechanical filtration. However, they do not remove nutrients from the water. Pond water is exposed to radiation from a UV lamp held inside a quartz tube. There are lamps of different wattage for various pond sizes; most require replacement every six months or so.