Garden Pond Filters and Filtration Systems: A Complete Guide

Filters and Filtration

If a pond is purely for plants and wildlife, a filter isn’t necessary—biological systems establish naturally. But if fish will live there, a filter should be seriously considered.

Why Filter?

To clean the environment. A pond is a self-contained ecosystem needing waste removal. As an artificial environment, it requires artificial methods to stay waste-free, enabling reasonable fish stocks. Before installing a filter, you need a suitable pump to force water through it. Consider pump and outlet positions relative to waterlilies—they don’t thrive in moving water. Reserve an area of still water for them.

Filtration Systems

Three major types: mechanical, chemical, biological.

Mechanical pushes water through gravel, clinker, or foam, catching suspended particles. Useful for stirred-up plants/compost, but doesn’t significantly improve water quality—only physical cleaning.

Chemical removes toxic/undesirable chemicals using absorbent properties of carbon and zeolite. Zeolite (natural ion exchange resin) removes ammonia in new systems, rechargeable with salt. Carbon removes chlorine, heavy metals, organic dyes, odours. Organic dyes (phenol, tannin) cause brown discoloration and can’t be removed mechanically.

Chemical filtration doesn’t replace biological; you still need a site for biological decay. If carbon stays long, it becomes biologically active; removing it reduces biological capacity, increasing ammonia/nitrite.

Mechanical and chemical filters can be combined or separate. With multiple chambers, fit a bypass pipe so one can be cleaned while the other functions. Chemical filters constantly improve—visit your aquatic supplier for latest info.

Biological (most popular) uses bacteria in filter medium to break down ammonia → nitrites → nitrates (readily used by plants). Long-term: beneficial bacteria take time to establish. They’re part of the pond’s ecology, vulnerable to water condition changes. Some pond cures damage them—check before buying.

In biological systems, anaerobic bacteria partially break down organic matter, releasing ammonia. In confined conditions, this kills fish, but aerobic bacteria convert ammonia to less harmful nitrites/nitrates, assimilated by plants. These beneficial bacteria need oxygen, living in oxygenated water passing down through the filter.

Mechanical filtration activates instantly (switch on pump). For long-term economy and success, biological is preferable.

Types of Filter

Internal: immersed in water, can be disguised with planting. Decide before pool construction to incorporate sympathetically; otherwise may be intrusive. Modern portable internal filters are easily disguised and connect neatly. But koi filtration needs likely exceed even the best portable internal systems.

External: pool stays uncluttered, but must be near pool, with regular maintenance access. Disguise with planting. If a rock garden is part of the feature, hide filter within it—plan from outset.

For effective operation, connect filter to a pump with more capacity than manufacturer recommends. Spare pump capacity is preferable to later discovering a complete pumping change is needed. However, flow rate through filter must match manufacturer’s specification. Pump size critical if also running a waterfall/fountain—much more powerful unit needed if filtration shares the role. Sometimes separate pumps for each purpose have advantages.

Filters need regular maintenance for efficiency. Frequency depends on filter medium and water condition, but weekly check in peak summer ensures safety.

Filter Media

Success heavily depends on filter medium. Diverse options, each with advocates.

• Gravel: widely used, available in grades; 6 mm (¼ in) works best. Irregular, porous grains give large surface area for bacteria and good straining.
• Processed lava granules: sponge-like, large surface area, capillary web. Efficient physical filter, plenty of bacterial space. Inert, sterile, neutral pH, light.
• Baked clay granules: similar to hydroculture medium. Clean, easy to use. Sand creates awful problems; only role is in sand-fed filters (koi keepers’ prerogative).
• Foam: popular, but clogs frequently, needs regular cleaning. Easy to clean. Only use filter-recommended foam; hardware-store foam may have fire-retardant chemicals that kill fish.
• Filter brushes: large bottle-brush shapes, fit snugly. Need regular cleaning; excellent bacterial homes.
• High-surface-area sintered glass: increasingly common, removes nitrate.
• Plastic mouldings: shaped plastic with large surface areas for bacteria. Common shape like plastic spring/hair roller (hair roller is acceptable substitute).

Gravel is widely used; baked clay and lava granules have large surface area; foam clogs but easy to clean; use only special filter foam; brushes popular and neat; sintered glass removes nitrate; plastic mouldings provide bacterial homes.

Natural Filtration

Plant filtration is trendy. Adapt to garden pool: establish a small subsidiary pond or filter box with vigorous plants absorbing phosphates, nitrates, etc., then run water through a conventional mechanical filter back to pool. Plants lock up excess mineral salts; periodic thinning removes chemicals from the cycle. Doesn’t remove significant ammonia/nitrite.

The Filter

Most pond owners should buy a ready-made filter suiting their needs, not build one—unless keeping large numbers of koi. Ready-made filters need careful placement for efficiency and unobtrusiveness. Custom filters are easier to accommodate but best avoided until experience or expert advice is gained.

Two modes: up-flow and down-flow. Up-flow: pipe leads water into bottom of filter chamber, permeates up through medium. Down-flow: water enters top, percolates down to outlet. In garden pools, little difference; down-flow easier to maintain (large debris collects at top). Both benefit from a pre-filtration chamber for large organic debris.

Under-gravel filter: adaptation of aquarist method—perforated pipes beneath gravel layer on marginal shelf. Needs constantly moving water for aerobic bacteria. Must be home-made; not for beginners. Efficient and easily disguised, but difficult to maintain.

Seasonal Adaptations

Each filter has an outflow discharging clean water. Summer: pump some distance from filter to create mixing currents improving quality. Return water above surface to aid oxygenation. Winter: move pump closer to filter and surface, return beneath water for smoother effect.