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Common Koi & Pond Myths

Updated: Tue, 10 Mar 2020

The truth about Koi & ponds. Water gardening and Koi keeping have become so popular in recent years, that there is unfortunately a lot of misinformation out there. As a pond and fish health specialist, I get a lot of questions regarding pond building and fish issues.
Common Koi & Pond Myths


By Dayleen Van Ryswyk aka “The Koi Lady”

Water gardening and Koi keeping have become so popular in recent years, that there is unfortunately a lot of misinformation out there. As a pond and fish health specialist, I get a lot of questions regarding pond building and fish issues. I love getting questions, because it means people are genuinely interested and want to learn. I’m a firm believer that when people are educated on a subject, they tend to make more informed decisions. The following, is a list I have complied of common pond & Koi myths based on questions I am most asked (they are in no particular order.)

#1 Koi do not grow to the size of their environment. As with most fish, they grow to the size their genetics dictate. They will grow slower in a small or crowded environment but they will eventually get to be 18”- 26” on average. If you would like them to grow faster, doing small water changes more often, will remove growth pheromones the fish instinctually give off in times of crowding. Most hobbyist’s probably notice this phenomenon when they take fish they have had for years in an aquarium, then put them out in a pond where there are more gallons of water. They suddenly see their wet pets rapidly balloon in size.

#2 There is no such thing as miniature Koi. This myth was probably created in the attempt to sell fish by unscrupulous dealers to un-educated hobbyists. Though, there are in fact jumbo Koi that can reach almost 4 feet in length. The biggest one I have ever seen was 42” long and almost a hundred pounds. It also came with a $70.000 price tag…ouch!

#3 Butterfly Koi were not developed by crossing a Koi and a Goldfish together. Koi are from a different genus than Goldfish and though they can interbreed, it always results in a sterile hybrid. Koi (Nishikigoi) were originally bred to the Indonesian long finned carp. The result was a long finned (Butterfly, Dragon Koi) in all the same varieties standard Koi come in. Though Butterfly Koi are extremely popular in North America, the Japanese still refuse to acknowledge them in any of their top Koi shows. I believe that as the younger breeders in Japan start taking over the family businesses, you’ll start to see Butterfly Koi in shows. Albeit, in their own classification and not with standard Koi.

#4 Goldfish are not hardier than Koi. Koi do in fact grow much larger than Goldfish. Because of this, their environmental requirements are greater. Neither species is really a cold water fish. They will tolerate periods of cold but if they are too long in duration or temperature, either fish can perish. Also, because Koi are so much larger, their oxygen requirements are higher. Being bigger also means more waste, more waste equals more filtration. Keeping large Koi in a small pond with out proper filtration is like keeping a Great Dane in a small yard. It can be done but it’s a lot more work and probably not so healthy for the dog (or fish.)

#5 You can’t have Koi if you want pond plants. Koi can be kept in heavily planted ponds with out destroying the plants. Understanding the behavior of your Koi can greatly reduce your stress load when it comes to keeping nice water plants. Koi are omnivores, which means they eat plants and animals. Koi are grown in mud ponds (ponds lined in clay) and their normal behavior is to root around in the mud looking for grubs and bugs. When you see them nose down rooting through your water lily pots, it isn’t because they are trying to annoy you. They are looking for bugs to eat. If you cover the soil of your water lilies with lava rock, the Koi won’t mouth it out, because they don’t like the texture of the lava rock in their mouths. Using rocks bigger than they can mouth out also works well. When your Koi start eating your plants they are trying to tell you that they need more greens in their diet.

We feed our Koi leaf lettuce, frozen peas and kale and they leave our plants alone. If your fish aren’t used to vegetables they may treat it like a predator and shy away from it. Once your most curious Koi tries it, they will all devour it. Incidentally, we also feed our Koi watermelon, oranges, grapefruit, strawberries, cooked chicken, cooked rice, pasta, brown bread, cheerios cereal, shrimp, freeze dried krill, worms and silk worm pupae in addition to their regular pellet food. The fruits are a great source of vitamin C for their immunity and the other foods provide valuable protein and carbohydrates. If they aren’t receiving a varied diet from you, then they will most likely take it upon themselves to eat what’s available, namely your plants!

#6 Goldfish cannot freeze solid and come back to life. I hear this one a lot in the fall at winterizing time. It sure would make life a whole lot easier on the pond owner if we could just bag up our fish into zip lock bags and place them in the freezer for the winter, then plop them back in the pond in the spring to defrost and come back to life. Sounds silly doesn’t it. I have by mistake, missed the odd Goldfish in a plant pond, only to find it frozen solid in the ice. I can honestly tell you, they have never come back to life! What I believe perpetuates this misconception, is the fact that Goldfish are pretty tough and resourceful and in an effort to save their own life, they will burrow down in the smallest of muck filled crannies to survive. The pond owner assumes the pond has frozen completely solid, (fish included) and comes to the conclusion that they can freeze and then come back to life. If you care about your fish, please properly winterize their pond.

#7 Ponds use a lot of water and can’t be included in low water conditions or with xeriscape landscaping. This can be true but doesn’t have to be. There are ways to have a pond or water feature that won’t make you a water hog. Obviously, I’m a water feature lover and having said that, I believe they can be put in pretty much any landscaping plan. Here are a few tips to help conserve water; keep any waterfalls to a minimum. The more splashing that occurs, the more evaporation there is. Maybe go for more of a meandering stream. Don’t have your feature out baking in full sun. If you keep it more in the shade, it will evaporate less. Invest in easy to maintain over-sized filtration. The reason being, the less you have to backwash or clean something, the less water it will use. Being over-sized means you’ll have to do less maintenance. Keep your fish load very low. Having less fish means less water/filter maintenance. Goldfish are beautiful but they are extremely prolific and spawn from early spring until fall and from a very early age. Maybe get only 2 or 3 Koi as they only spawn once a year after they are 3-5 years old. Either that or keep your fish all same sex, this is easier said than done. Do keep fish tough as they will help with the biological aspect of the pond and keep mosquito larvae from turning your pond into a hatchery.

#8 I don’t need filtration for my pond, I’m building a “natural” pond. My usual answer to this one goes kind of like this. Unless you’re building a truly natural pond, as in, mud/clay bottomed with a natural stream entering and exiting, it isn’t a natural pond. There is absolutely nothing natural about a rubber or cement lined man-made hole in the ground. In nature you have plant filled streams and wetlands that filter these natural ponds. The water flows in and out constantly, keeping the water clear and removing nutrients.

In our man made ponds, we don’t usually have water flowing in and out constantly. This is where filtration acts like Mother Nature in a way. In a well designed pond you have several elements of filtration. Usually, you have a pump that draws water in and takes it to a filter (either in the pond or out) the water goes through the filter to remove particles and mechanically clean the water. That’s called mechanical filtration. In the filter there is usually also a media that is used for biological filtration. Typically it is a type of ribbon or plastic balls that the good bacteria in your pond colonize on. This good bacteria is what helps remove harmful excess nutrients from your water. Remember to never clean your bio-media with chlorinated water as it will kill your good bacteria.

When the addition of a stream is added to your pond, it helps your ponds health in a few ways. One, a stream works like a wetland in nature. The plants in the stream (this includes the algae) absorb excess nutrients from the water, which keeps the plants green and healthy. The rocks in the stream work like the biological media in your filter, because the good bacteria also colonize on them and help break down and consume excess nutrients. The water gets oxygenated as it tumbles down the stream and water with high oxygen content makes every living organism in the pond healthier. The water, the fish, the plants, the filtration, everything works together to create a healthy eco-system. Take away any one thing and it’s really difficult to find a harmonious balance. Even the dreaded algae are a good thing. A nice fuzzy coating of it on your pond liner is one of the healthiest things for your fish and pond. If it’s growing excessively, it’s because you are out of balance with not enough filtration or plants in comparison to your fish load and are probably feeding too much. The same can be said for green water.

So, when people say they want to build a natural pond, I really think what they mean is that they want to build a “more natural” pond. Just remember, no matter what filter you have or how diligent you are maintaining your pond. You will inevitably have periods of time that you’re growing more algae than your fish can eat. After all, this is a pond and not a swimming pool. The weather changes, your fish load changes, things change and throw things a little out of balance, that’s the truth. There are ways of dealing with little issues that arise that are more natural than others. So, before you throw in a bunch of chemicals to kill algae and throw your whole eco-system out of whack, speak to a pond professional about why things are out of balance. It could be as simple as “how” you clean your filters.