A koi pond rich with biodiversity is perhaps an ultimate goal for every pond enthusiast, as maintaining one can be both challenging and rewarding. Many factors affect the success of a koi pond, especially because the growth of koi is dependent on water depth, pond size, climate and weather conditions, and even on other fish in the pond. Exciting enough as a sole focal feature, koi gradually increase in size and can even live for up to 40 years.
If you intend to grow these alongside a host of other fish species, it’s important to keep a few things in mind. First, select for species that have similar pond requirements. These species will normally be found in wild temperate habitats such as rivers, streams, and lakes. As a rule of thumb, however, opt for those that you can find in a reputable fish or aquarium store to ensure that you don’t bring diseases into your pond.
Second, bear in mind that koi tend to explore all levels of a pond. This means that they will inevitably interact with other fish, regardless of where they prefer to reside in the water column. Though peaceful in nature, koi may be aggressive towards some smaller species of fish (e.g., guppies, danios, mollies, and minnows), especially in an overcrowded pond. As opportunistic omnivores, they may even eat their own fry. These scenarios are quite uncommon, however, and should not occur if your koi are stocked with appropriate species and in a comfortably large set-up.
Koi Pond Mates
1) Goldfish (Carassius auratus)
Native to East Asia
The typical aquarium species, Carassius auratus is now so widely bred that there are hundreds of well-developed breeds to choose from. Like koi, goldfish belong to the carp family (Cyprinidae), but differ from their cousins due to their lack of mouth barbels. Goldfish can vary greatly in terms of appearance and color, and just about every type of variation has been explored for its robustness and longevity. The most common type, which is often what young children find themselves growing in fish bowls, is single-tailed, red to orange in color, and largely inexpensive. In contrast, the rarest types can be described as having the most bizarre features, such as bubble-shaped eyes and large egg-shaped bodies, and can sell for thousands of dollars!
The requirements of goldfish may vary depending on the breed, but most store-bought varieties are hardy and can do well in outdoor ponds. Goldfish thrive in water temperatures ranging from 65 – 85˚F (18 – 30˚C). Though they are a cold-water species, they can survive in heated ponds as long as there are cooler pond sections that are cast in shade. To maintain a water profile that is suitable for goldfish growth, utilize a filtration system that can prevent the buildup of ammonia and nitrates. Preferably, concentrations for these nutrients should be kept at zero.
One of the benefits of growing goldfish alongside your koi is they have similar ecosystem requirements and can consume the same types of food. Koi can even consume goldfish flakes, which have the same ingredients as koi pellets. You may also find goldfish schooling with koi and vice versa. They come from the same family after all!
2) Grass carp (Ctenopharyngodon idella)
Native to East Asia
Do you have difficulty controlling algae and weed growth in your koi pond? If your answer is yes, then this fish may be the answer to your problems. Another member of the carp family, Ctenopharyngodon idella is more popularly known as grass carp because of its propensity to consume aquatic plants. In fact, this species was brought to the US and Europe precisely for this reason – to control the growth of aquatic vegetation in lakes, ponds, and canals. In controlled numbers, this species can offer the same ecosystem service to your koi pond, though it is advisable to be wary and regularly monitor the growth of your prized ornamental greens.
Found in turbid rivers and lakes in the wild, this species is highly adaptable to a variety of growth conditions and can reach lengths of 100 cm (39 in) in enclosed settings. With a lifetime of 5 – 9 years, grass carp are able to regularly consume enough plant material to support up to 40 pounds (18 kg) of body weight in their adult years. Luckily, this species has a top-down approach to consuming plants, which means they won’t tear out plants from soil and cause your pond to be murky.
Also cultivated as an economically important food source, grass carp is now one of the most highly produced species in aquaculture systems. Do keep in mind that, though these are widely cultivated, this species should not be grown in a pond if there is a risk of escape. Because of its hardiness, it has the potential to become an invasive species in your area and may cause ecosystem disturbances as they compete for food with wild fauna.
3) Suckermouth catfish (Hypostomus plecostomus)
Native to northeastern South America
A prized species in the aquarium trade, the suckermouth catfish is just one of many pleco species that can provide a vital service for your pond. This particular fish has earned its name because of its ability to latch onto surfaces with its mouth, as a strategy to “suck” off algae. For this reason, this species and its near-relatives are often referred to as janitor fish. Having just a few of these may be an effective strategy to control algae growth along the walls and floor of a small pond.
In the wild, this armored catfish is found in actively flowing streams and rivers, and normally above rocky substrates. This rough natural habitat is testament to the strength and hardiness of this species. Though a pond normally has standing water, you need not worry about this catfish requiring a stronger water flow. They can grow to be quite comfortable in an enclosed set-up and can even mature to lengths of 15 inches (38 cm) over the course of 10-15 years. As juveniles, they tend to be peaceful when kept in groups. As they age, however, they may become aggressive to one another, so it’s best to limit their numbers in a pond.
Suckermouth catfish will thrive in a variety of pond conditions, and can even withstand harsher nutrient concentrations in water.Do bear in mind, however, that the water quality itself must still be good, particularly if you intend to raise this species alongside koi. As catfish will typically stay on the bottom of your pond and will not require any special types of fish food, it is an ideal pond mate for koi. Note that these catfish are nocturnal, and they will often be found hiding in crevices or under rocks during the day.
As this is a tropical species, they do require a warm climate and water temperatures between 22-28°C (72-82°F), so are best suited as pond mates to koi in countries with very mild winters (unless you can bring them indoors into an aquarium!).
4) Redear sunfish (Lepomis microlophus)
Native to southeastern United States
Commonly known as redear sunfish, Georgia bream, chinqapuin, and sun perch, Lepomis microlophus is a vigorous and attractive addition to a garden pond! Though there are many sunfish species that can thrive in an enclosed set-up, the redear sunfish is the most ideal for growth alongside koi. This eye-catching species has orange to cherry-red markings on the operculum, which is a flap of tissue that protects the gills and looks like a fish’s ear. In ponds, lakes, and reservoirs, redear sunfish are often found congregated in areas that are afforded protection by logs or aquatic plants.
Every now and then, snail infestations can occur in outdoor ponds. The good news is this species is perfect for preventing this! Yellow grub, a particularly nasty snail that can parasitize on pond fish, is just one unwelcome pond visitor that the redear sunfish can eliminate. It’s no wonder this species is also known as the “shellcracker”. In ponds, it is normally found foraging close to the bottom of the water column, as it has a preference for warmer water and a gentle, if not absent, current.
This hardy fish should be added as an individual to your pond, unless raised alongside carnivorous fish, as they can reproduce very quickly and take over your pond. They reach sexual maturity in their second year and will create spawning nests, basically depressions along the bottom if your pond has soil, above which they will spawn in late-spring – early-summer. This shy species may rarely approach the surface. Be sure to keep your pond water clear so that you can catch a glimpse of this elusive wonder!
5) Largemouth bass (Micropterus salmoides)
Native to Central and Eastern US, Southeastern Canada, and Northern Mexico
If you’re up for a rewarding and educational experience, the largemouth bass is a great fish to grow with your koi. Care for this species may be more demanding compared to other species on this list, but it should be fully manageable so long as a few requirements are met. Also called widemouth bass, Florida bass, and southern largemouth, this species is sure to be an eyecatcher as it slowly meanders around your pond. With a lifespan of 10-16 years, this economically important fish can grow up to 75 cm (29.5 in) and weigh up to 11 kg (25 pounds).
Largemouth bass will require a larger pond area to grow comfortably. Compared to koi, it will also require a higher oxygen content in the water, so a waterfall feature or a reserved pump would be ideal to ensure proper aeration. Bear in mind that this species is a carnivorous fish, and will require live food. Ideally, to prevent your bass from preying on koi and other ornamental fish, your pond should have a thriving population of spawning fish. The largemouth bass will be instrumental for keeping populations in check by feeding on fish spawn and larvae. Smaller fish, such as minnows, bluegills, and sunfish will also satiate your ‘pet’ largemouth.
If possible, have only 1 – 3 individuals of this species in your pond, and make sure to allocate hiding places to ensure maximal comfort. For this purpose, driftwood, rock caves, and waterlilies can be used to decrease instances of aggression. Another consideration to keep in mind is the largemouth bass’ special requirements for overwintering. Compared to koi, largemouth bass may need access to deep warm water, depending on where your pond is located.
6) Chinese high-fin banded shark (Myxocyprinus asiaticus)
Native to China
Experts at hiding, this shapeshifting species is not actually a true shark. They only possess their high dorsal fins while they are juveniles! Shockingly, they can look very different as adults – almost like totally different species. When purchased from pet shops, young individuals of this pseudo-shark have alternating black and white bands. As they reach lengths of 12 – 14 inches (30 – 36 cm), they gradually change in color and lose their bands entirely. Sexually mature males tend to be reddish in color, whereas females adopt a dark purple hue. Over the course of 5 – 6 years, this species can even grow to be more than 2 feet long, making it an ideal option for an outdoor pond.
Chinese high-fin sharks will likely choose to stay at the bottom of your pond, and would ideally prefer a high-quality substrate such as fine gravel. This species will require well-kept conditions, and may be prone to diseases in poor-quality water. As a cold-water species, it thrives best in temperatures between 55 – 75 ˚F (12.8 – 24˚C). It is also sensitive to water pH and hardness, favoring conditions often observed in freshwater aquariums (pH 6.8 – 7.5; 4 – 20 dGH).
If you incorporate juvenile sharks into your koi pond, make sure to provide many hiding places like driftwood, aquatic plants, and rocks. It is also advisable to provide them with open space, however, as they will quickly navigate across the center of your pond bottom. Moreover, they will appreciate ample water flow, along with high-protein food choices to complement algae (which they typically consume). Don’t fret if you are unable to find them a few days after releasing them into your pond, as they may just be hiding from your koi!
7) Orfe (Leuciscus idus)
Native to northern Europe and Asia
An attractive addition to your pond with unique behavior to boot, don’t think twice about adding Orfe to your koi set-up if you are able to purchase them in your area. Sometimes referred to as “Ide”, this torpedo-shaped ornamental species is built for speed and can be very hyperactive at times. Orfe prefer to stay just under the water surface and like to go about their day in shoals. If you’re lucky, you’ll spot a few of them dart out of the water, as though to signal other fish in their group. Nonetheless, do watch out for orfe jumping outside of your pond as this adventurous species is notorious for trying to escape!
There are three varieties that are typically grown in ponds, and these include the golden orfe, silver orfe, and blue orfe. Of the three, the golden orfe is the most popular one and can grow up to 2 feet (61 cm) long, depending on the size of the pond. In nature, this species is normally found in clear cool waters, with temperatures ranging from 13 – 25˚C (50 – 77˚F). Typically, orfe like to eat protein-rich snails, insect larvae, and fish fry in the wild. In ponds, their diet can be supplemented with meat-based pellets and mealworms.
With proper pond conditions, orfe can live up to 20 years. Regularly maintain your pond’s water quality and ensure that ample aeration is provided at all times. Watch this fish thrive alongside your koi for years to come, as they are friendly, fun, and timelessly fascinating!
