Garden Pond Filtration: Mechanical, Biological and UV Stages

How garden pond filters work: mechanical, biological and UV stages, gravity versus pressurised designs, sizing by volume and fish load, turnover, maintenance and winter operation.

How pond filtration works

A pond that holds fish accumulates solid debris and dissolved nitrogenous waste. Fish excrete ammonia mainly through the gills, and the Southern Regional Aquaculture Center (SRAC) describes it as a stressor that reduces appetite and growth and kills fish at high concentrations. Filtration therefore has two core stages: a mechanical stage that traps solids and a biological stage where bacteria convert ammonia to nitrite and then nitrate. An ultraviolet (UV) clarifier is an optional third stage against green-water algae. A wildlife pond without fish needs no pump, filter or UV (Practical Fishkeeping).

Mechanical stage

The mechanical filter removes solids before they clog the biological media. Wikipedia lists the forms used on koi ponds: vortex or settlement chambers, brushes, matting, sand or gravel beds and sieve screens. Box filters use sponges graded coarse to fine; Practical Fishkeeping notes that modern sieves are easier to clean and catch finer particles. On koi ponds a bottom drain carries heavy solids to the filter and a surface skimmer collects leaves, pollen and floating uneaten food. Mechanical media need regular flushing or backwashing, because trapped waste keeps decomposing inside the filter.

Biological stage

SRAC describes the nitrification sequence: Nitrosomonas bacteria oxidise ammonia to nitrite, and Nitrobacter bacteria oxidise nitrite to nitrate. Both steps consume oxygen. Nitrite is toxic, while nitrate is non-toxic at the levels usual in recirculating systems. The bacteria colonise every wet surface, but a biofilter concentrates them on inert media (plastic, ceramic, rock) with a large surface area per unit volume. Wikipedia lists aerated designs (moving-bed, trickle or shower) and non-aerated ones (sand, cross-flow, bead). Nitrification produces acid, so SRAC recommends alkalinity near 150 mg/L and pH 6.8–7.2. The University of Florida IFAS states that a new biofilter needs six to eight weeks to mature. Bacteria settle on clean surfaces, so clogged media lose capacity whatever their quality.

UV clarifier

Ultraviolet light makes free-floating algae flocculate (clump) so the mechanical filter can remove them, and it kills free-floating bacteria and some pathogens (Wikipedia). A UV unit cannot work alone: it depends on the pump and filter to remove the damaged cells (Practical Fishkeeping). Contact time is decisive, so the flow through the unit must match its rating. The lamp should be replaced every six months, or once a year before the spring–summer season when the unit runs seasonally.

Gravity-fed and pressurised filters

Box (gravity-return) filters sit at the pond edge above the water line. A pump feeds water over graded sponges and plastic biological media, and clean water returns by gravity. Most have a bypass against overflow when sponges block. They offer larger capacity and less frequent upkeep.
Pressurised filters are sealed canisters. They can be placed away from the pond, partly buried, or used to push water uphill to a waterfall. They suit small to medium ponds but need more frequent cleaning, often by hand-washing sponges.
All-in-one submersible units combine pump, media and often a small UV lamp. They suit small ponds and water features, but the whole unit must be lifted out for maintenance.
Gravity-fed koi systems use bottom drains and skimmers instead of an in-pond pump. Water flows through rigid pipework into large settlement and biological chambers similar to fish-farm filters, with air pumps supplying oxygen.

Sizing by volume and fish load

Sizing starts with pond volume from average length, width and depth. Practical Fishkeeping warns that manufacturer ratings often assume no fish: halve the rated volume for goldfish and halve it again for koi, so a filter rated for 20,000 litres serves 20,000 litres with no fish, 10,000 with goldfish and 5,000 with koi. For green-water control the same source recommends a system rated for one and a half to two times the actual volume, since undersized systems matched to inaccurate volume estimates are a common cause of failure. Tropical Fish Hobbyist describes a filter rated for 6,000 US gallons as adequate for no more than five koi, because koi are heavy-feeding, heavy-waste fish.

Turnover rate

Turnover is how often the pump passes the whole pond volume through the filter. Practical Fishkeeping recommends at least once every two hours, so a 1,000-litre pond needs at least 500 litres per hour reaching the filter; green-water algae can divide every two hours on a sunny day, and slower turnover lets them outpace the UV. Wikipedia, describing koi ponds, gives at least once per hour. UF/IFAS adds that any fish pond needs a pump to move and aerate the water.

Maintenance cadence

Rinse biological media only in pond water; tap water carries chlorine or chloramine that kills nitrifying bacteria.
Replace media gradually: no more than one third at a time, at weekly intervals (Practical Fishkeeping).
Flush or backwash the mechanical stage regularly.
Test ammonia, nitrite, pH and alkalinity routinely; SRAC recommends sampling at the same place and time so trends are comparable.
Replace the UV lamp every six months, or yearly before the growing season.

Seasonal operation and winter shutdown

Koi enter dormancy at about 10 °C and pond fish stop eating in cold water. Tropical Fish Hobbyist reports that switching filters and waterfalls off completely for winter led to fish losses; it recommends gentle circulation of about one turnover every ten hours instead of the summer two hours, with the outflow fully submerged. Practical Fishkeeping adds: switch off fountains and air pumps, because cold water already holds more oxygen (UF/IFAS: 11.9 mg/L at 45 °F/7 °C against 7.4 mg/L at 90 °F/32 °C) and spraying it into freezing air super-chills the pond; divert returns below the surface; raise the pump intake 1–2 feet (30–60 cm) off the bottom to keep the warmer layer; insulate pipes; and bring the UV unit indoors, since frost can crack it. Keep a gas-exchange hole in ice with a pan of hot water or a floating heater, not by hammering. UF/IFAS advises leaving clean-outs until February or March to spare overwintering wildlife. In spring an unfed biofilter has lost capacity, so feeding should rise slowly (SRAC), and waterfalls should stay off until the water reaches 60 °F (16 °C), because splashing over cold rock can cold-shock fish (Tropical Fish Hobbyist).