Aquaculture: Keeping koi fish for fun and profit

Question: I’m interested in keeping koi fish. Can you help with more information, please?

Having koi in the garden pond has become a popular hobby, but if you want to breed the fish as well, it’s important to know how to go about it before you start.

Koi breeding is a delicate process and if the basics aren’t in place, you could suffer financial losses. According to Tienie du Preez, an experienced koi breeder, it’s essential to have enough good quality water and an excellent breeding pair.

Whether you keep koi for your own enjoyment or to sell commercially, they must be free of disease. A superior koi must have a good body shape and colour, and the correct colour placement. “These factors determine the quality of the fish and the price,” he says.


  • Plenty of water is needed for large-scale koi breeding.
  • Du Preez has more than 30 ponds and needs approximately 1 million litres of water a month to sustain his operation.
  • Koi need acceptable water conditions to thrive.
  • Use borehole water if you have access to it, but it’s not advisable to use chemical-rich municipal water, unless it’s been treated.
  • Water that has a pH close to neutral is ideal.
  • Frequent full and partial water changes are needed to maintain water quality.
  • Breeders and hobbyists are advised to change 15% to 20% of the ponds’
    water every week.
  • Remember always to maintain a supply of oxygen to the koi, especially during water changes.
  • Using a biological water-filtering system is crucial.
  • The system removes all dissolved wastes that accumulate in the ponds. If they aren’t removed, they could kill the entire koi population.
  • Consider installing a UV light as well, as part of a filtering system. This device controls green water and helps to disinfect the water supply.
  • If you run pond water through a UV system before it re-enters the pond, the level of harmful bacteria in the pond will be reduced.


  • Use plastic, earth or concrete ponds, depending on your budget.
  • All koi ponds need to have the correct “contour” (shape), otherwise you’ll struggle to keep them clean.
  • Koi pond designers explain that leaves and other debris will collect in “dead” areas (where there’s no water movement).
  • The position, depth and size of the ponds will depend on the breeding needs and property layout.
  • Plastic ponds are the least expensive. You can buy a ready-made pond, or hollow out a piece of ground and line the hole with thick 500 micron PVC plastic.
  • The benefits of a plastic pond are that it is fairly easy to install and can be stocked almost immediately.
  • Concrete ponds are permanent – and expensive to build.
  • The price of cement is high, and building the pond is labour-intensive. It also requires steel reinforcement.
  • Earth ponds are the most natural. “Koi are exposed to natural trace elements and beneficial minerals and bacteria,” says Du Preez.
  • Make sure that your soil type is suitable for constructing earth ponds – a clay-type soil works best.
  • You’ll need to install a bottom drain to transport waste to the filter. Without it, the filter cannot perform properly and it will be difficult to maintain water quality.
  • Some breeders prefer to heat ponds in winter, but this could be costly.
  • The only benefit is that the koi will grow faster.
  • Koi are least active in the colder months.
  • Spawning (breeding) only takes place once a year.


  • Iodised coarse salt can be added to the water, which reduces stress levels and controls ammonia concentrations and parasites in the water.
  • Make sure that the concentration is not too high. Three kilograms per 1 000 litres of water is adequate. Too much salt could burn the koi’s skin and belly.
  • Koi are prone to diseases such as white spot, as well as worms and lice, but these can all be kept under control by using vaccines and antibiotics that can be administered orally through sprinkles on food.
  • Frogs are also a problem. Not only do they swallow the fry (newborn fish), but they’re also able to transfer diseases, as they are carriers of harmful bacteria.
  • This typically happens when they move between ponds.
  • Make sure your fish are kept in tip-top condition during winter because when the breeding season comes they have to be in the best possible shape.


  • Breeding koi for commercial purposes shouldn’t be motivated solely by profits – you also have to love it.
  • The market is tough to get into and a breeder has to do extensive marketing to be successful.
  • The retail price of koi can be deceptive, as it’s not an indicator of the price a supplier will receive. The size and age of the fish determine price.
  • Possible outlets include selling to specialist stores, pet shops or directly to the public.
  • Feed costs should also be carefully planned for in the budget.
  • The fry (newborn fish) feed on their shells for the first three days after hatching, but after that young koi may be fed Daphnia, a live bait food source.
  • Older fish could be fed pellets, which vary in size for the various age groups.

Also read: Aquaculture: Fish farming offers possibilities

  • This article was written by Wilma den Hartigh and first appeared in Farming SA.

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