Why filtration is more important than the pond itself
Koi are messy, heavy-feeding fish. They produce significant waste. Without a properly sized and designed filtration system, ammonia from fish waste builds to toxic levels within days. This is why most failed ponds fail — not because of bad fish or bad luck, but because the filtration was undersized, wrong type, or simply absent.
The three filtration types — all three are needed
Mechanical filtration
Physically removes solid particles — fish waste, uneaten food, leaves, debris. Works like a vacuum cleaner. A pond skimmer removes surface debris before it sinks. A settlement chamber or drum filter removes fine solids from the water column. Removes visible particles only — not dissolved toxins.
Biological filtration
The most critical type. Uses colonies of beneficial bacteria (Nitrosomonas and Nitrobacter) to convert toxic ammonia from fish waste into nitrite, then into relatively harmless nitrate. Without this conversion, fish die. Bacteria live on the surface of the filter media — the more surface area, the more bacteria, the more capacity. Never clean biological filter media with tap water — chlorine kills the bacteria.
UV sterilisation
Water passes through a tube containing an ultraviolet bulb. UV light disrupts the DNA of algae cells and harmful microorganisms, preventing them from reproducing. Controls green water outbreaks and reduces pathogen load. Not a replacement for biological filtration — works in addition to it. UV bulbs need replacing every 12 months even if they appear to be working.
How to size your filtration system
The rule: your filter should be rated for at least twice your pond volume. A 5,000-litre pond needs a filter rated for 10,000+ litres. This is because filtration ratings are calculated under ideal conditions — lower fish stocking, cooler water, lower feeding rates than most real-world situations. In Australian conditions, err on the side of significantly more filtration.
Turnover rate: the entire pond volume should pass through your filtration system at least once per hour. For ponds with higher fish stocking or in warmer climates, aim for two to three full turnovers per hour.
Gravity vs pressurised systems
Gravity systems: water flows by gravity from the pond into filter chambers, then is pumped back. More efficient, easier to clean, better suited to serious koi ponds. The filter must be positioned at or below pond water level.
Pressurised systems: water is pumped through a sealed pressurised filter. More compact, easier to position flexibly. Better suited to smaller ponds and ecosystem setups. Requires more frequent cleaning under pressure.
The nitrogen cycle — understanding what's happening in your water
Fish produce waste → waste produces ammonia → beneficial bacteria convert ammonia to nitrite → different bacteria convert nitrite to nitrate → water changes and plants remove nitrate. This cycle takes 4–8 weeks to establish in a new pond. During this period (called cycling) fish are vulnerable. Never overstock a new pond — start with fewer fish than the system is designed for and allow the bacterial colonies to establish.
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