Koi vs Goldfish – Which Fish Is Right for My Pond? (Main Differences)

Koi vs Goldfish Differences – Which is The Best Fish For Garden Ponds?

As a new pond owner you may very well feel a bit overwhelmed by all of the options out there and the dynamics involved in creating your own idyllic decorative fish pond. As koi and goldfish are two most common species found in ornamental ponds, we’ll try to help you explore which one may be more suitable for you and your individual needs as a new pond owner!

A Short History of Ornamental Fish & Varieties 

1) Ornamental Koi Carp

Koi carp originate from Japan, and have been bred for their large variety of color and patterns, such as “Asagi” and “Beko” – pictured.

Koi are a domesticated subspecies of the common carp, Cyprinus carpio. Originating in Japan, Koi, or Goi as they are commonly called in their homeland, literally translates to “carp.” Typically bred for their unique, diverse array of beautiful colors and patterns, they add a certain flare to any ornamental pond. While naturally bottom-feeders, this freshwater species is omnivorous (meat & plants) and will feed at any depth where food is available. They do exist in natural ecosystems, though wild koi are now listed as a vulnerable species in need of protection, primarily due to human activities altering their habitat and diminishing the flooded areas that are crucial for spawning. To help keep the staggering amount of koi variations straight, they are separated into 13 basic classes, with different sub-types depending on pattern and color differences.

Modern koi are now available in over 100 different varieties, offering an incredible diversity of colors, patterns, and scale arrangements to choose from. Regular breeds don’t differ much in terms of size or shape, although there are “jumbo” varieties of each breed which corresponds to a larger size and superior breeding.

These larger types are highly desired by many hobbyists, however, they’re more expensive and will require more pond space and extra feeding for optimal health. For most pond owners, regular sized koi are more than enough, as regardless of size, they will still retain the same beautiful patterns and colors.

2) Ornamental Goldfish

Although smaller than koi, goldfish also come in a large variety of shapes, colors, and patterns, such as the ‘Vieltail’ – pictured.

Goldfish were also originally domesticated from Asian carp, and are now bred for their vast vairety of shapes and patterns. Their colors can vary greatly, though not nearly as much as their close fishy cousin, the koi. Initially bred in ancient China over a millennium ago as a symbol of harmony and good fortune, goldfish were gradually transported to Japan, Portugal, Europe, and North America, acquiring great popularity in each location. Over time, multiple subspecies of goldfish have been cultivated, providing a multitude of options for size, shape, coloration, and pattern – which is also why they’re now also called “fancy” goldfish.

Hundreds of years of careful breeding to obtain various desired traits has resulted in roughly 200-300 recognized goldfish breeds. There are some variations in color and patterning, though not as much as that of koi. However, in contrast to koi, goldfish breeds do provide a myriad of options in terms of body size and shape, fin size and shape, eye configuration, and tail length. They’re also smaller, which means they’re easier to care for in a smaller pond environment.

While modern goldfish have been found in the wild (likely due to humans placing them there rather than naturally existing and breeding in these natural locations), they are considered only a domesticated species and generally do not survive for long outside of captivity.


Size, Hardiness & Lifespans of Koi & Goldfish

1) Ornamental Koi Carp

Koi are much larger than goldfish, but can withstand low temperatures and will hibernate during winter.

Koi grow an average of two centimeters per month, often-times reaching a maximum length of over 3 feet (sometimes considered “jumbo”). When properly taken care of, most live to be approximately 50 years old – a key fact to keep in mind when deciding whether or not to have them in your pond – but they can live to be over 100, and the oldest known koi lived to be 226!

They are a fairly resilient species, able to resist many parasites and diseases that are known to impact numerous other fresh water fish. However, they are susceptible to koi herpes virus (which many are born with), and survive the best in a pond environment with low waste, high aeration, and stable water parameters. The biggest threat to koi are sudden changes in water quality and their environment, so keeping things stable is one of the most important aspects of koi keeping. Water quality should be tested regularly to ensure optimal parameters are met and there are no sudden changes.

Since koi are much larger than goldfish, you likely won’t be able to bring them indoors during the winter. However, koi can tolerate very low temperatures without problem, and will enter a state of hibernation as the water temps begin to fall. So long as you have taken care of the pond during the build up to winter (a process called “winterization“), koi can survive fine outdoors all year.

2) Ornamental Goldfish

Goldfish are much smaller than koi, and may need to be brought indoors over winter if the pond is shallow.

Goldfish, on the other hand, have more variation in size and shape depending on the variety. Some may be only a couple of inches in length, and at most will reach a foot long. Often times, goldfish are associated with a short lifespan. This is more often than not an unfortunate result of inadequate care, as goldfish can easily live an average of 10 or more years in good conditions!

They are also considered to be quite tough so long as water is properly filtered and oxygenated, and in outdoor ponds can withstand surface ice formation for short durations (though it is recommended to transfer them indoors if it’s possible that temperatures will drop that low), or install some winter protection equipment to be safe.

In general, the ideal temperature range for goldfish is 68 to 72 degrees Fahrenheit. While able to tolerate fluctuations in pH levels better than other fish species, a pH of 7.2 to 7.6 is best. Many fancy goldfish varieties are more prone to illness and disease than koi, but again this is primarily due to deficient care or being kept in too small of a space, such as a fish bowl.

Of all the goldfish breeds, the ‘common’ goldfish is the most hardy and can survive fairly well outside in a similar fashion to koi. Other fancy goldfish breeds which have been overly bred for variety, would do well being brought indoors when harsher weather arrives, such as in winter.


Fish Pond Size – Can You Actually Keep Them?

1) Ornamental Koi Carp

Koi will be most comfortable in larger ponds, but can survive in smaller environments if water filtration is high capacity.

Since koi can grow to be rather sizeable depending upon the variety, your pond size and how many fish you decide to have should be based on their average adult size and not their size at the time of purchase (they will grow – fast!). A general rule is 1 inch of fish for every 10 gallons of water, but because koi add significantly to your pond’s bioload (waste levels) due to their larger size, you should have no more than 4 koi per 1,000 gallons of water. Still, many argue that 1,000 gallons is simply too small to allow adult koi to move about and live comfortably, and an even larger would is more ideal.

Although you may see a large amount of koi packed into a small space when you go to purchase them, the breeders likely have massive filtration systems in place to handle the large bioload. Likewise, the koi won’t be in that small environment for long, so it should have little negative effect on their health. If you intend to keep koi personally, and don’t want to spend thousands on expensive filtration equipment, you need to make sure your pond is large enough for them to thrive for the rest of their lives!

2) Ornamental Goldfish

Fancy goldfish, such as Pearlscales (pictured), can be kept in small ponds or large aquariums due to their size.

Contrastingly, goldfish are of course smaller than koi and naturally require less space, though pond size guidelines are somewhat contingent on the goldfish variety. As before, the commonly accepted rule is 1 inch of fish per 10 gallons of water. This is still commonly debated, however, as all fish are different.

Smaller fancy varieties of goldfish seem to do well with 1 fish per 10 to 20 gallons, while larger breeds such as the common goldfish and comets need at least 20 gallons per fish. Again, all fish are different and you should make certain that you conduct plenty of research before choosing a particular pond size or goldfish breed(s). For instance, pearlscales (pictured) are considered a small to medium sized fancy goldfish breed and yet require a minimum of 30 gallons of water per fish to flourish because they generate a surprising amount of waste.

Conversely, if you like the idea of having fancy goldfish more than having an outdoor pond, it may be easier (and cheaper) to simply purchase a large indoor aquarium instead. This would make cleaning and maintenance easier, and they’d already be nice and cosy during the winter.


Ongoing Pond Fish Care – Considerations, Maintenance & Extra Costs

1) Pond Depth, Temperature & Climate

De-icers help keep an air hole open in the ice over winter, and should be installed in any pond that gets surface ice.

Koi should be kept in ponds with water that is at least a meter deep, as this will ensure they have enough room to swim and water does not easily freeze in winter. If you live in a location that experiences colder temperatures, you should ensure that your pond is closer to 1.5 or more meters deep, as the last thing you want is a pond that freezes solid. Deeper water also does not experience temperature fluctuations as quickly, with it providing better insulation for fish during hibernation. In most cases, you’ll want to install a surface heater (pictured) during winter to keep a hole open in the ice so gas change can take place, as oxygen can run dangerously in icy ponds. Finally, having a deep pond for koi ensures they’re able to hide more easily from predators, such as herons, or when they’re stressed and need some time away from the group. Basically, if you’re considering koi as your pond fish, the deeper the better!

This same concept applies to goldfish, though in temperate conditions they only require a pond depth of about half a meter. In cooler climates, a meter deep should suffice to prevent the water from freezing totally (unless you live in a particularly harsh climate). Some varieties of goldfish can survive outdoors in winter, simply congregating at the warmer, unfrozen bottom of the pond and becoming more sluggish until spring arrives. With all of this in mind, if you’re constructing a garden pond in an area with winters that will regularly cause water to freeze, goldfish may be a more suitable option as they are smaller than koi and can more easily be transferred to an indoor tank during frigid times. Most goldfish species can be placed into a large aquarium or holding tank until warmer weather in spring arrives, which makes them a little easier to handle in very cold climates in comparison to koi carp.


2) Water Filtration & Equipment Expenses

Both goldfish and koi will require a pump and filtration system, with the filter working to remove harmful waste substances.

As previously mentioned, both koi and goldfish are best suited to particular pH levels and water parameters. Water quality should be tested regularly (once per week at approximately the same time of the day is recommended), and affordable test sticks can be purchased that will detect levels of ammonia, nitrites, nitrates, and pH all at once by simply dipping the stick into the water. Any level of ammonia or nitrite can be fatal to koi and goldfish, and both of these toxins are more likely to be present in water with higher pH levels or ponds containing too many fish. A proper filtration system is vital to the health and survival of your pond as a whole, and aquatic plants can also be strategically used in ponds as they will help absorb excess nitrogen and nutrients, thus preventing deadly overdose (eutrophication).

If you’re looking to purchase koi, or a large amount of smaller fish, you will also need to invest in a more efficient pump and water filtration system. The more fish you add to the pond, the higher the bioload, and the more filtration you’ll need to keep the eco-system within safe parameters. As well as the higher equipment purchase price, larger equipment costs more to run per month, so you should also factor future costs into your purchase budget. Here is a short list of the kind of equipment you’ll need to consider when adding both goldfish and koi to a garden pond:-

  1. Pond Pump (to power filtration system)
  2. Filter Box (to remove waste)
  3. UV Clarifier (to reduce algae & harmful bacteria)
  4. Aerator/Air Pump (to oxygenate water)

3) Feeding Frequency & Nutrition

High quality feeds are essential for optimal fish health, and should be a part of every fish keepers routine.

Koi should be fed two to four times per day in summer months, which may equate to quite a lot of food depending on the size of your stock! Food pellets should also be high quality for optimal health, containing 30-40% aquatic sourced protein, healthy fats, low ash, and a wide vitamin and mineral profile. Many commercial off-the-shelf feeds you find in stores are not good quality, which is why they’re so cheap in price. For the best health, you should always consider a high quality feed designed for optimal health, immune support, growth, and color – this will ensure your pond fish are thriving, not just surviving! Check here for out top koi feed guide.

As with koi, the standard goldfish feeding frequency is two to four times per day, but should decrease jointly with temperature. As opportunistic feeders, goldfish will continue to eat if food is provided to them – a useful mannerism in the wild where food is not always abundant and there exists competition and natural selection, but deadly in captivity. Standard goldfish food is rich in carbohydrates but relatively low in protein, similar to commercial koi feeds. Again, you should opt for a higher quality feed which provides all the ingredients goldfish need to grow strong and healthy. Next to maintaining good water quality, having a good quality feed is probably the second most important aspect of optimal pond keeping! Check here for out top goldfish feed guide.

Overall, the feeding and water quality requirements of goldfish and koi are similar. The main detail to bear in mind is that koi are larger than goldfish. As such, they require a more sizeable pond, more food, produce more waste, and necessitate stronger filtration equipment. This all equates to more monthly costs and initial investments!


4) Cleaning, Maintenance & Water Changes

Having fish means you’ll need to regularly clean the pond and equipment, which may include the use of nets, water changes, or vacuums.

While goldfish are considered by some to be hardier than koi, their needs vary more widely depending upon breed. Although smaller than koi, they produce a greater amount of waste in relation to their body size and as such require more dissolved oxygen (and more frequent pond cleaning). The more fish you have in your pond, the more often water quality should be tested and the more aeration and filtration you’ll need to provide. And with more equipment, comes more responsibilities..

In general, your water filter should be cleaned out at least once per week to ensure optimal, efficient functioning. In the spring, a more thorough cleaning may be needed. This consists of safely removing the fish and any plants, draining the pond, scrubbing and/or power washing the liner and any rocks, removing any organic and artificial debris, cleaning out the pumping system, and so on. Water changes are also essential for ponds and fish, as they replenish water alkalinity (kh) which acts as a pH buffer against dangerous fluctuations. Adding fresh water will also increase the amount of trace minerals, such as calcium and magnesium, which both koi and goldfish need for healthy bodily functions (absorbed directly through the gills). The frequency and size of water changes will depend on the type of fish you keep or the size of your stocks, with koi requiring more frequent water changes than small amounts of goldfish.

This all needs to be taken into account before adding fish, as if you’re not able to keep up with cleaning and regular water changes you’ll need to instead keep your fish stocks low, or opt for just a small number of goldfish for minimal bio-load and less frequent maintenance.


So, Which Fish? Final Thoughts..

Koi offer a greater number of color and pattern options than do goldfish, and their size and overall shape does not change much from variety to variety. They’re relatively easy to take care of with a bit of research and practice, though their larger size means that they need more food and a bigger pond, have a more significant bioload on the system, and stronger equipment is needed to properly maintain water quality.

Goldfish do not offer nearly as wide of a selection of colors and patterns, but their body size, shape, and configuration does differ from breed to breed. Because many of the breeds are so different, goldfish can be more difficult to properly take care of than koi (though they are by no means overly challenging, as even novice fish enthusiasts can have great success with them). Their smaller size correlates to a smaller pond, which in turn equates to easier pond maintenance despite their proportionally greater waste production. They are notably cheaper than koi as well and able to withstand cooler temperatures and temporary bouts of surface freezing, but also more prone to disease and illness.

Overall, if your pond is less than 1,000 gallons, or you’re new to the hobby and are more budget conscious, goldfish would likely be a more compatible option for you. Alternatively, if you’re willing to spend a bit more money, are looking to have a larger pond, live in a more temperate area that is less vulnerable to freezing temperatures, and prefer more ornate and unique fish, koi are a good match for you and your pond.

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