A koi pond needs three kinds of filtering: mechanical (catching dirt), biological (bacteria breaking down waste) and UV (clearing green water). The pump should move the whole pond volume through the filter roughly every one to two hours. Clean the filter in pond water, never all at once, and replace the UV bulb about once a year.
Why koi ponds need serious filtration
Koi grow big, eat a lot and produce a lot of waste. A pond that looks fine with a few small fish can turn cloudy and smelly as they grow. Most problems with koi ponds come down to a filter that was sized for the pond on day one, not for the fish five years later.
The three jobs a pond filter does
- Mechanical filtering. Brushes, sponges or mats catch leaves, waste and debris. This is the part that clogs and needs rinsing.
- Biological filtering. Good bacteria living on the filter media turn ammonia from fish waste into much less harmful nitrate. This is the part you must not destroy when cleaning.
- UV clarifying. A UV light inside a clear sleeve treats the water as it passes, clumping free-floating algae so it can be filtered out. It clears green water but doesn't remove string algae.
Choosing a pump
- Flow rate. For koi, aim to move the full pond volume through the filter about every one to two hours. A 10,000 litre pond needs roughly 5,000 to 10,000 litres per hour at the filter.
- Head height. Pumps lose flow the higher they push water. Check the flow at your actual height, not the number on the box.
- Running cost. The pump runs 24 hours a day, so an efficient model can pay for itself in electricity.
- Backup. A battery-backed air pump keeps oxygen in the water during power cuts.
Skimmers and bottom drains
A surface skimmer pulls floating leaves and debris into a basket before they sink and rot. A bottom drain draws waste from the lowest point of the pond into the filter. Both mean less work and cleaner water, and they're much easier to plan into a new pond than to add later.
Cleaning without killing the bacteria
- Rinse sponges and mats in a bucket of pond water, never under the tap.
- Clean part of the media at a time, not all of it in one go.
- Don't switch the filter off for long periods. The bacteria start dying within hours.
- Clean the UV quartz sleeve when you change the bulb. A dirty sleeve blocks the light.
Water tests for koi ponds
| Test | What you want |
|---|---|
| Ammonia | 0 |
| Nitrite | 0 |
| pH | Steady, roughly 7 to 8.5 |
| KH (carbonate hardness) | Not too low, so the pH can't crash |
Readings drifting in the wrong direction month after month usually mean the pond needs more filtration, fewer fish or less food. The plain-English version of what these tests mean is in water testing and the nitrogen cycle.
When to call us
If your pond water won't clear, the pump keeps blocking, fish are gasping or you're planning a new koi pond, we can help. We maintain ponds across Cape Town on a regular schedule, and we can check whether your pump and filter are the right size for the fish you have now.
Regular pond servicing
Pumps, filters and UV checked and cleaned, water tested and treated, fish checked. Priced per pond.