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Koi are the jewels of the garden pond. They're intelligent, long-lived and can become tame enough to feed from your hand. They're also large, hungry fish that produce a lot of waste, so a koi pond needs more space and better filtration than a typical goldfish pond. Get those basics right and koi can live for 25 years or more.
Koi care at a glance
| Need | Recommendation |
|---|---|
| Pond size | At least 4,500 litres (1,000 gallons), ideally much more |
| Depth | At least 1.2m (4ft) in part of the pond |
| Stocking | Roughly one adult koi per 1,000 litres |
| Filtration | Rated well above your pond volume, with UV |
| Water turnover | The whole pond volume through the filter every one to two hours |
| Feeding | Adjust by water temperature (see below) |
| Lifespan | 25–35 years, sometimes longer |
Is your pond big enough?
Koi grow quickly, commonly reaching 45–60cm (18–24 inches). A small fish bought at 10cm can outgrow a garden pond within a few years. A deep pond also keeps temperatures more stable through summer heat and winter cold, and protects fish from herons. If your pond is under about 4,500 litres, goldfish, shubunkins and orfe are usually a better choice.
Filtration: the most important investment
Koi produce far more waste than goldfish. A koi pond filter needs three stages:
- Mechanical: sponges, brushes or a sieve that trap solid waste so it can be removed.
- Biological: media where beneficial bacteria convert toxic ammonia and nitrite into nitrate.
- UV clarification: a UV clarifier keeps the water free from green algae so you can see your fish.
As a rule, choose a filter rated for at least twice your pond volume when keeping koi. Pair it with a reliable, energy-efficient pump, and don't forget running costs: it will run 24 hours a day. Read our guides to pond filters and pond pumps, and browse our pond filters, pond pumps and UV clarifiers.
Add aeration
Koi need plenty of oxygen, especially in warm weather and at night. An air pump such as the Evolution Aqua Airtech 75 is a simple, low-cost safeguard.
Water quality
Most koi health problems start with water quality. Test regularly, weekly in summer, for ammonia, nitrite, nitrate and pH.
- Ammonia and nitrite: should always be zero.
- pH: 7.0–8.5, and stable.
- Water changes: replace around 10% a week in the growing season. Always dechlorinate tap water with a product such as Tetra Pond AquaSafe, Eco Pond Chlorine Guard or Nishikoi Dechlorinator. See all pond dechlorinators.
When starting a new pond or after cleaning, a bacteria starter such as Evolution Aqua Pure Pond helps the filter establish.
Feeding koi through the year
Koi are cold-blooded, so their metabolism follows the water temperature. Feed by the thermometer, not the calendar.
| Water temperature | Food | How often |
|---|---|---|
| Above 20°C | Growth or colour-enhancing pellets | Two to four small feeds a day |
| 15–20°C | Staple pellets | Two to three feeds a day |
| 10–15°C | Wheatgerm or easily digested food | Once a day |
| 5–10°C | Wheatgerm only, in small amounts | Every few days on mild days |
| Below 5°C | Don't feed | — |
Only feed what your koi eat in about five minutes. Quality koi foods such as Nishikoi Staple Pellets are a good everyday diet, and Nishikoi Growth Pellets suit warm summer months. For autumn and spring, use Tetra Pond Wheatgerm Sticks. Browse our full range of pond foods or read our pond fish food guide.
Buying and introducing koi
- Buy from a reputable dealer, and look for active fish with fins held upright, no ulcers, red streaks or raised scales.
- Ideally quarantine new koi in a separate, filtered and aerated tank for three to four weeks. Koi can carry viruses such as KHV that don't show symptoms until temperatures change.
- Float the bag for 20–30 minutes to match temperature, then gradually mix in pond water.
- Add only a few fish at a time so the filter can keep up.
Spotting health problems early
Watch your koi every day at feeding time. It's the easiest health check you can do. Signs that something is wrong include:
- Fish hanging at the surface, gasping, or sitting at the bottom with clamped fins.
- Flicking and rubbing against the sides, which often means parasites.
- White spots, cloudy patches, ulcers, red streaks or raised scales.
- Not coming up to feed in warm weather.
Test the water first; it's the cause of most problems. Treatments such as Tetra Pond MediFin tackle common parasites and infections, and API Pond Melafix supports healing of minor wounds. See our pond fish treatments. For ulcers or serious illness, consult a vet experienced with koi.
Protecting koi from predators
Herons can empty a koi pond in a morning. A pond cover net, steep sides, deep water and places to hide all help.
For year-round routines, read our guides to preparing your pond for winter and clearing green water and blanketweed.
