Japanese kois, an ornamental carp species, are fish that look particularly handsome in a garden pool because of their bright colors. Clear water is therefore essential, otherwise the beauty of these exotic fish is not fully visible. After a short acclimation period, kois become so tame that they let you touch them, and they even like to take food from people’s hands. Kois recognize individual people by their gait and try in various ways to attract their attention, sticking the head out of the water, for instance, or begging for food by making loud eating noises, or practically turning somersaults in the air.
What You Should Know: These fish are not cheap, and setting up a pool to meet their needs adequately also runs into some money. Kois also grow big¡ªup to 24 inches (60 cm)¡ªand consequently need plenty of room. Two or three kois can easily be kept in a goldfish pond or an ornamental pool with other fish, but if you would like to keep a larger number you should offer these splendid animals a pool of their own, where they can really thrive.
– A koi pool should measure at least 100 square feet (10 m2), or preferably 150 square feet. A deep-water area of about 10 square feet (1 m2) and with a depth of 60 to 70 inches (1.5-1.8 m) is recommended if the fish are to overwinter in the pool.
– Kois have no need for a marshy zone, and any marsh area should, in fact, be separated from the pool by rocks or set up in a way that keeps the fish from swimming through it. But you should plan to have a marshy area all the same because water attracts all kinds of creatures that can find food and shelter only in a marshy environment.
– Clean, oxygen-rich water is essential for kois. Kois consume protein-rich food in considerable quantities and produce corresponding amounts of waste. Fish excretions have a negative impact on the water because they cannot be broken down through natural processes. That is why you need a good filter with as large a filter volume as possible and a powerful pump that turns over about 500 gallons (2,000 L) per hour. A coarse bio-filter material (no cotton wadding!) is recommended. You will also have to install an air pump (membrane pump) with an air stone. The air pump should be left running continously all year around.
Location: Choose a partially shaded site (with five to six hours of sunshine) close to the house. Full sun should be avoided because it stimulates excessive algae growth, which makes it more difficult to keep the water clean.
Bottom Sealing Materials: Pool liners, preformed fiberglass shells of sufficient depth, or concrete poured by a professional contractor all are good bottom sealers.
Plants: Water lilies of all kinds are best, but they should be planted in containers. With their big leaves water lilies help shade the water below them, and the underside of the leaves also removes large amounts of nutrients and carbon dioxide from the water. Algae are therefore unlikely to become a problem in the vicinity of water lilies.
Other floating plants that fit in well with water lilies are floating heart, water soldier (Stratoites aloides) and, especially, cattails (Typha latifolia), which absorb a lot of organic waste materials.
Submerged plants like waterweed (Elodea canadensis) and pondweed (Potamogeton) are even more efficient at cleaning up waste materials than water lilies. They are planted in containers with a layer of pebbles on top of the soil so that the kois don’t dig them up too quickly.
Animals: You can buy kois at pet stores or from dealers specializing in fish. The price depends on the quality of the breed and on the markings, especially those visible from above. These markings are supposed to be as symmetrical as possible. There are many different koi varieties that differ from one another in color and markings. Some are solidly colored in yellow, orange, silver, or gold; the multicolored strains include red-white-and-black, white-and-red, and blue-and-silver with a red dorsal stripe, to mention a few.
When you first stock a pool measuring 100 square feet, eight to ten juveniles are plenty because koi grow quite fast. A 1-year-old koi measures about 4 to 5 inches (10-13 cm), but by the time the fish is 3 years old it will have grown to about 14 inches (35 cm).
The Pool Margin: The pool’s edge has to be cat-proof; otherwise, these trusting fish fall victim to felines all too easily. Shallow parts of the pool should be blocked off with rocks to keep the koi out so that cats tempted to catch them would be forced to get wet.
Except in the marshy area, you will want to have borders around the pool that you can walk on. Flagstones or concrete slabs extending about 8 inches (20 cm) beyond the water’s edge are a good idea.
Upkeep: During the summer all that is required is feeding the fish and occasionally thinning plants that grow too profusely. The water quality should be checked regularly and, especially with small pools, a monthly water change is recommended (replace 30 percent of the water).
Ordinary goldfish or pond fish food is not adequate for kois, but pet food manufacturers also market special koi foods. If you want to offer your fish a special treat, you can give them pieces of beef heart (without gristle) soaked in a concentrated multivitamin solution.
In the summer it is best to feed koi fish several times a day, but only as much as they will eat up right away. When the water temperature drops to around 54¡ãF (12¡ãC) in the fall, stop feeding.