Pond Mates for Koi
Potentially Good Pond Mates
Sturgeon (Acipenser sp.)
Members of the Acipenser genus of sturgeon are viable candidates as pond mates for koi. We recommend the sterlet sturgeon (Acipenser ruthenus) due to its smaller size compared to its massive cousins like the white sturgeon (Acipenser transmontanus) which can also be commonly found in the pet trade. Sturgeon are bottom dwellers with downward facing mouths and can be tricky during feeding; we suggest pushing high protein sinking foods to the bottom. It is important to remember that sturgeon are not algae eaters and will not clean the pond.
Golden Shiners (Notemigonus crysoleucas)
Commonly found as bait fish, the golden shiner is a small hardy North American cyprinid that can adapt to a wide range of temperatures. Shiners will readily breed and multiply in numbers quite quickly and can act as dither fish for the koi and other inhabitance of the pond. Small shiners and fry can of course be predated by koi and caution should be taken when adding shiners to ponds with butterfly/longfin koi as they may or may not nip their fins. Golden Shiners are not typically found in fish stores, but are commonly found in bait shops; however, it is imperative that bait shop shiners are medicated and quarantined no matter what as they are not intended as pets and can carry all sorts of parasites and diseases.
Goldfish (Carassius auratus)
Goldfish are tricky as many of the fancy goldfish breeds are likely not compatible with koi as they will have a difficult time competing for food. When choosing goldfish as pond mates for koi, we advise to pick larger breeds that have a more streamlined body. Of course, no two fish are the same, and you can always take a risk and try the more delicate breeds. Monitor the fish daily, and if problems occur, remove the fish immediately.
Chinese High-fin Banded Shark (Myxocyprinus asiaticus)
The high-fin shark is a commonly found cypriniforme in the pet trade sold alongside koi, goldfish, and other coldwater fish. Do note that the black bands and coloration will disappear as the fish matures reaching an adult size of around 4 feet. High-fin sharks may have a difficult time competing with koi during feeding; the high-fin shark will not thrive or reach adult sizes relying on algae alone.
Questionable Pond Mates
Channel Catfish (Ictalurus punctatus)
The channel catfish is a New World catfish commonly found in North America. They are usually found as their albino variants at local fish stores; do caution that this is a large catfish growing to around 4-5 feet in length and can easily consume small to medium koi whole.
Bullhead Catfish (Ameiurus sp.)
The bullhead catfish is a small to medium North American catfish; although compared to its larger relative the channel catfish, this genus is usually not found in tropical fish stores. Despite their size, they can potentially be aggressive and harm even large koi; their sharp spines can also harm the human handler if caution is not taken.
Amur Catfish, Japanese Catfish (Silurus asotus)
The Amur catfish or Japanese catfish is a species of Eastern Asian catfish belonging to the group sometimes referred to as sheatfish; this species can come in a motted black/brown and white. While they do not grow nearly as large as their European cousin the Wels Catfish (Silurus glanis), the Amur catfish can grow up to 4 feet long and can easily consume small to medium koi whole and may or may not harm larger koi with their row of teeth.
Common Plecostomus (Hypostomus sp. | Pterygoplichthys sp.)
Plecostomus are a group of armored South American catfish known as loricariids. While it is enticing to add an algae eater to your koi pond, it is generally ill-advised to keep koi and plecostomus together indefinitely. Plecos can be kept with koi during warmer months but should be taken out during cold months; in vise versa, koi should not be kept in permanently warm water. In indoor aquariums with stable temperature, a middle ground where both species can thrive is certainly achievable. It is also important to know that most plecostomus are omnivorous and may end up sucking the slime coat or inflicting wounds on koi if they lack sustenance; the common pleco is no exception and will become less effective as an algae eater at it ages and matures craving more meaty foods.
Crayfish (Procambarus sp.)
We generally do not recommend adding crayfish to a koi pond; we have first hand experience of the damage they can inflict upon baby koi especially when their numbers reach the thousands in a mud pond. The most common and biggest offender of this is the Louisiana swamp crayfish (Procambarus clarkii); this crayfish is invasive across the world and can have a quite nasty temperament; we are unsure of how well the electric blue crayfish (Procambarus alleni) commonly found in tropical fish stores as freshwater “lobsters” act in the same environment. Marbled crayfish (Procambarus virginalis), otherwise known as marmorkrebs or cloning crayfish may be the most viable candidate due to its more peaceful nature. Do note that all crayfish will devour and decimate plant life in your pond or aquarium. Please research your local laws regarding which species of crayfish, if any you can possess as they have the potential to cause environmental havoc.
Probably Bad Pond Mates
Temperate Monster Fish
Aggressive and dangerous fish should be avoided such as snakehead (Channa sp.), gar (Atractosteus sp. | Lepisosteus sp.), bowfin (Amia calva), pike (Esox sp.), bass (Micropterus sp.), and other predatory fish. Some of these fish may be illegal in various states and should be reported accordingly to their respective laws if seen.
Most Tropical Fish
As mentioned with the common pleco, tropical fish and koi generally do not mix due to the differences in temperature requirements. However, if you live in a climate where both species can thrive all year long, there are possibly many viable options available to you. Due to the vast number of tropical fish available in the market, use your judgement to determine what is probably okay and what would potentially lead to disaster. For example, a ripsaw catfish (Oxydoras niger) is likely to perform better than a redtail catfish (Phractocephalus hemioliopterus); this could be akin to comparing a sturgeon to a channel catfish.