Golden Garden: Add Koi and Goldfish to Your Pond for a Pop of Color

Author: Lea Maddocks

Koi and goldfish make colorful and hardy additions to a pond during the spring and summer months, and one pond expert offers her tips for choosing the right fish for your particular situation.

Dive into Fishkeeping

For the northern hemisphere, summer is now in the air. To complement the colorful budding flora returning to our gardens, why not take your hobby outdoors and add a different splash of color with koi or goldfish?

History of Ornamental Pond Fish

As many aquarists may already know, our hobby was essentially born from keeping ornamental ponds and pond fish—a trend that began its popularity during the Tang Dynasty (618–907 BCE) in ancient China and has since gained popularity around the world. Orange and gold mutations of the common olive-green Prussian carp (Carassius auratus gibelio)were the first ornamentals keptand, over time, were bred into domesticated goldfish (C. auratus auratus). These domesticated variants now sport many color, body, and fin morphs to please the eye.


Interestingly, it is noted in some books that the original ornamental goldfish varieties, particularly those with long, flowing caudal fins, were bred specifically to be viewed from overhead, i.e., in a pond or outdoor container. In essence, it seems these morphs are deigned to be best enjoyed as ornamental pond fish. Goldfish dominated as ornamental fish for hundreds of years, being popular across China, spreading in the 1600s to Japan, and reaching Europe and the United States by 1850.


Koi (which means “carp” in Japanese) are a considerably more recent development in pond fish. Koi are derived from the common carp (Cyprinus carpio), which was originally aquacultured for food both in China (during approximately 500 CE) and in Europe by the Roman Empire (27 BCE–400 CE). These fish also spread to Japan, where they were also aquacultured for food for many hundreds of years. As with goldfish, color mutations occasionally arose, and in the early 1800s, these were developed via selective breeding into colored koi, officially dubbed “nishikigoi” (“brocaded carp”). These colored koi quickly gained popularity as ornamental pond fish throughout Japan in the early 1900s. From this beginning, koi began spreading throughout the world as a beautiful feature fish in larger ponds and water gardens.

How to Introduce Fish to Your Pond

  1. Once your pond is fully set up and running, stock it with pond plants and marginal plants, and then let it mature for several weeks to two months in colder climates to allow it to biologically cycle.
  2. Regularly testing for ammonia, nitrite, and nitrate will let you know when it is ready to introduce fish. Most small ponds will get going with bacteria introduced with pond plants, but you can supplement this with the cycling bacteria that are commercially available.
  3. Once cycled, add your fish one to two at a time, wait a week or two between increasing the stock, and monitor your metabolic wastes at all times. As with all fish, acclimate slowly, adding a small amount of water to the bag at 5- to 10-minute intervals, and release your fish once the bag is over 90 percent pond water. Do not add water from the bag to the pond to reduce the risk of introducing disease. Once nitrates are stable at 20 to 30 ppm (40 ppm maximum), your pond is fully stocked.

Keeping a Goldfish Pond

Available in various colors and shapes, goldfish are ideal inhabitants for smaller ponds. Bright orange or albino comets and shubunkin varieties with their calico markings do particularly well in ponds, as they are hardy, can grow quite large (up to a foot), and often have long, flowing tails to admire. These fish are able to survive most normal outdoor temperatures, though a source of shade to keep the pond below 80°F in summer is recommended and a heater to keep the pond over 50° in winter will reduce stress and be welcomed. Goldfish will go into a period of reduced activity or dormancy at low temperatures, resting at the base of the pond, moving and eating little, if at all. This is normal, and their liveliness will return when temperatures rise again in spring.

Pond Requirements

Fancy goldfish can also thrive in a pond environment, though they should not be kept in a place where the temperature swings greatly (i.e., a small pond in the sun, which is prone to temperature fluctuations, or at temperatures above 80° or below 60°). Fancy varieties are also more sensitive to waste, so keep your nitrates in check. In terms of space, you should allow at least 15 gallons or more per adult fish and ensure good oxygenation of the water. Goldfish can dig up some plants, so plants with thick stems, such as established lotuses, lilies, reeds, rushes, and floating plants, should be used with these fish.


Feed a high-quality pellet or flake staple with supplemental feedings of frozen or live foods, ideally two to three times a day. They will relish some blanched leafy greens and frozen or live meaty foods as well as nibble at pond plants and snack on aquatic insects for additional nutrition.

Kept well, goldfish can live in your pond for well over 15 to 20 years. If you have a very large pond, you can also keep goldfish and koi together.

Keeping a Koi Pond

Pond Requirements

If you have a large pond, you have the option of keeping koi. Koi can grow very large, up to 3 feet, and they can weigh up to 40 pounds. Like goldfish, they are social creatures, so you will need to house at least three to five. At least 200 to 300 gallons per koi is recommended, though some have had success with smaller ponds with excellent filtration. Nevertheless, it is best to provide them with the swimming space they need to grow and thrive. A 1,000-gallon pond would be sensible for a small koi pond housing three to four koi. Long and wide ponds at least 6 to 8 feet across will provide adequate swimming space, and some deep areas (4 feet or more) are also recommended for koi to overwinter in. As with goldfish, koi will reduce activity and become dormant at low winter temperatures.

Plants and Feeding

Koi are also generally more destructive to plants, so you may need to use more periphery plants in your water garden and possibly mature water lilies in large, heavy pots that cannot be knocked over. Koi also prefer to be fed two to three times a day. A specific koi staple food is a must, though supplementing with peas, pumpkin, oatmeal, wholemeal bred, broccoli, spinach, beans, carrots, oranges, watermelon, and insect larvae (which they can obtain naturally outside) are excellent for treats and conditioning.

Growth and Lifespan

If you buy small koi, which is the cheapest way to obtain them, understand that the most growth occurs in the first two years, with fish growing in length on average 6 to 7 inches a year. This slows to 4 to 5 inches in the third year. Their maximum adult length will usually be reached at 10 to 13 years old. Pond size and health will dramatically influence the adult size of koi, with some reportedly growing over 4 feet, but most reach a size of 2 feet and a weight of approximately 15 to 22 pounds. Remember that koi can live over 30 to 40 years, and some are rumored to have lived well over 100 years. If you have a smaller koi pond and want a bit more life and color but are too short on space for more koi, you can add goldfish. They will generally coexist peacefully. Koi come in many color patterns, and by joining your local koi club, you might be able to find some rare or interesting varieties to add to your pond.

Pond Plants

Your climate and local nursery availability will dictate what pond plants you are able to keep for your pond, though the variety is wide and most hobbyists will be spoiled for choice. Most plants fall into the category of totally submerged plants, plants with surface leaves but with stems rooted in the substrate or in pots, free-floating plants that live and replicate on the surface, bog plants for planting in the shallows at the edge, or marginal water plants that like moist conditions near the pond but not necessarily in it. If you are after something unique, you may be able to order it through your nursery, via mail, or online.

Whatever plants you choose, consider how and where you will plant them, how they will spread, if they will require additional fertilization, pruning requirements, and other seasonal care. The most popular water plants used for most hardy ponds include water lilies, lotuses, reeds, rushes, water iris, elodea, and water lettuce, though your nursery will be able to steer you through the various types and variations within these species. Indeed, a variety of water lilies or lotuses alone can look striking.

When keeping fish such as goldfish and koi, remember that sturdy plants are best, as these species are prone to eating and uprooting plants. Koi will even knock over potted plants on planting shelves. To prevent this, try placing large stones around root bases or using floating plants. Both fish will benefit from nibbling on plants, and as free-floating plants usually regenerate fastest, use of these to some extent is encouraged—though do take note that plants like duckweed can be very difficult to remove once in a pond. Using a net to scoop out large amounts of this now and then may be needed to clear the surface so you can see your fish!

General Pond Maintenance

Maintenance will generally be determined by your pond’s size, stock, and filter system. However, most ponds will require some attention at least once a month. General maintenance will involve removing overgrown and rotten plants, leaves, and stems; cleaning out filters and skimmers; siphoning loose debris from the bottom; and conducting a partial water change to a level required to keep ponds clear and nitrates in check (usually 10 to 25 percent). And, as with aquariums, regular testing of pH, ammonia, nitrite and nitrate is strongly recommended to ensure you pond’s health is intact. Nitrates should remain under 40 ppm, ammonia and nitrite at 0 ppm.

pH Requirements

A pH of 7.2 to 7.5 is optimal for most goldfish and koi, but they can tolerate a range from 6.5 to 8. Addition of aquarium salts containing calcium is also recommended to harden the water to levels appreciated by goldfish and koi, which are approximately 100 to 250 ppm or 6 to 13 degrees GH to improve their osmotic function. Raising carbonate hardness over 100 ppm or 6 degrees KH by adding bicarbonate is also suggested to improve pH stability and counteract any pH drops from decaying mulm at the bottom, which can produce nitric acid. Naturally, keeping parameters stable is the prime objective for any pond. In a newer pond, larger and more frequent changes may be necessary while it matures.

Seasonal Maintenance

Season will also play a part in maintenance. Winter will require trimming back any plant die-off, ensuring any pond heaters are working well before frost sets in, and cutting back or even stopping feeding as fish become less active or dormant. Spring and summer will require increased feeding to match fish activity levels for growth and/or breeding. Plants may require the addition of fertilizers or pruning back if they grow excessively, insect pest activity should be monitored and addressed if need be (though fish will often relish mosquito larvae), and aeration should be checked to ensure it is adequate for keeping warmer water well supplied with dissolved oxygen. Autumn may require more regular clearing of fallen leaves from the surface and, again, adjusting of food supply to a level matching the activity of fish.

Algae Issues

Should you encounter algae issues, check to see if the nitrate and phosphate levels are elevated. If so, ensure the filters are clean and functioning well and, if need be, reduce feeding or stocking levels. Also make certain that the pond is not receiving too much direct sunlight. Algae is more common in ponds subjected to excessive sunlight, overstocking, and maintenance errors.

It should be noted that while algae can at best be unsightly, it can at worst be fatal. If a large bloom suddenly occurs, it can potentially photosynthesize so rapidly during the day that it can supersaturate the water with oxygen, causing embolisms in fish similar to the bends in divers, which can be fatal if they occur in sensitive tissues like the eyes or brain. Additionally, the algae respires at night, causing large CO2 build-ups that can cause deaths in the morning if aeration is inadequate. Algae blooms can also die off rapidly after consuming pond nutrients, quickly fouling the water with a large load of decaying organic matter that can then cause ammonia spikes followed by elevated levels of nitrite and nitrate. Ensuring a good regime of proper pond maintenance will keep algae-causing waste levels low and keep your pond clean, clear, and healthy.

Enjoy Your Goldfish and Koi

Now that you're familiar with the basics, you should be able to make a great start in adding goldfish or koi to your pond. With some careful forethought and proper planning, you will be rewarded with a beautiful water garden and a fabulous feature to enjoy for many years. So, northern hemisphere readers, spring into ponds this spring and have a great time taking your hobby to the garden—it makes me envious that I'm overwintering in Australia!

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