Koi fish in poind

Koi and goldfish are known for their playful and docile nature, which has made them beloved by pond owners for centuries. However, at times you might notice some unusual behaviors, and these can be important to understand. Often your fish are conveying a message through their actions.

Koi and goldfish are cousin species, so many of their behaviors signal similar things. However, koi are more sensitive to their environments, so they are likely to exhibit behavioral changes in a shared pond.

Here are some common and uncommon behaviors you may see from koi and goldfish and what they mean.


Fish will occasionally chase one another throughout the pond and possibly nip and bite at each others’ fins. This is normal behavior for koi and goldfish during the breeding season, but at other times might indicate that there’s a problem with your pond setup.


The primary reason you will see some of your koi or goldfish chase others is that they are spawning. Fish typically spawn in late spring or early summer when water temperatures are between 65 and 75 degrees. A male goldfish will target an egg-bearing female and chase her around the pond to encourage her to release her eggs. This can go on for several hours but will cease once the female accomplishes this.


If your koi or goldfish are chasing outside of the breeding season, check the ratio of pond space to the number of fish. A koi pond should contain at least 1,000 gallons of water, with 200 gallons per additional koi and 20 gallons per additional goldfish. A koi pond needs to be at least three feet deep but potentially much deeper depending on the climate. Goldfish require a minimum depth of two feet and possible depths of up to five feet. Living in an overcrowded pond can spark aggressive behavior in these otherwise docile species, as they compete for space and resources in a limited environment. Larger fish will tend to chase smaller fish away from their perceived territory.

Boredom and Stress

Koi or goldfish might display aggression as a result of boredom or stress. A pond with only water is an unnatural environment for koi and goldfish. Incorporating plants give koi and goldfish places to explore and treats to nibble on. Rocks also provide environmental variety and create hiding spots from predators. When pond fish have nothing in their surroundings that feels natural to them, they can become skittish and occasionally aggressive.

Poor Water Quality

Stress from poor water quality can trigger breeding. If your koi or goldfish are spawning in the off-season, or the chasing behavior continues longer than it should, it could indicate something wrong with your water parameters. Maintaining a healthy pond requires routine water quality checks. You should test water for pH levels, as well as ammonia, nitrites, and nitrates, all of which can harm fish when out of balance.


As a pond owner, you want to view your gorgeous koi and goldfish as much as possible. However, there are times when the fish might act shy and hide under rocks or plants. This could be from temporary factors that will easily resolve on their own, or it could indicate some changes you need to make to keep the fish more comfortable.

A New Pond

Moving to a new home requires an adjustment period for humans and fish alike. Your koi and goldfish will need time to get used to their new environment before they are comfortable. During this phase, you might notice that some of your fish aren’t so easy to spot. They might hang out on the bottom of the pond or under plants or rock structures. This is normal, and they should be swimming around fearlessly within a couple of weeks.

New Fish

When you add fish to your pond, it might take everyone a while to adjust to having new roommates. Naturally, the new fish will be nervous and might find the existing fish intimidating at first. The fish already living in the pond can become wary, too, causing them to retreat to their favorite hideaways until everyone warms up to each other.

Water Changes

Any change to the chemical balances in the water can stress fish, which could prompt hiding behavior. In addition to testing water quality regularly, you’ll also want to keep your water well-aerated to be sure the fish get an ample supply of oxygen. Also, check your filtration system regularly, cleaning or replacing filter media as needed.


Persistent hiding can be a sign that predators are making frequent visits to your pond. During an attack, fish will instinctively retreat to the deepest point of the pond for shelter, and they may remain there longer each time unless the threat is removed. Several options exist for deterring predators, and the best strategies depend on the types of animals that inhabit your neighborhood. Motion-activated scarecrows and netting are some universal options that help keep critters away.

Leaping or Gasping

A fish out of water is never a good thing. Leaping refers to when the koi or goldfish jump out of the water. This happens much more with koi than goldfish, and several reasons for it can exist. Gasping is when koi or goldfish stick their heads above the water surface in an attempt to get more oxygen. This almost always indicates a problem in the pond.

Not Enough Oxygen

Leaping or gasping at the pond’s surface could mean there is too little oxygen in the water. Consider various oxygenating options. A bottom drain is useful in koi ponds because it pulls the water with the lowest oxygen content to the surface to replenish. Koi ponds with only a skimmer and no bottom drains will typically have problems with aeration because the water at the bottom never circulates to the top. Air pumps and air stones can help, or you can combine a bottom drain and an air stone. Perform a partial water change, clean the filters, and clear debris. Oxygenating pond plants will also naturally add more oxygen to the water.

High Ammonia

High ammonia levels can burn fish’s gills, causing them to jump to escape the pain. The buildup of waste in the pond causes elevated ammonia levels. Management efforts can include cutting back on feeding, increasing aeration, adding beneficial bacteria, ensuring optimal filtration, and performing regular water changes.

Exploring and Having Fun

Koi are known for their athleticism and playfulness. Jumping can sometimes be a regular part of their routines, and it can be fun to watch as long as you have a system in place to keep them from getting out of the pond. If you have jumpers, installing pond netting will help keep your fish safe and also help protect against predators.


Flashing consists primarily of koi or goldfish dashing around the pond, rubbing their scales on rocks and the pond’s edges, and swimming on their sides.

Shipping Stress

Symptoms of shipping stress will occur for new koi as soon as they get out of the bag and into your pond. The good news is this is almost always temporary. Koi and goldfish are hardy, and they typically recover quickly from shipping stress — sometimes even within minutes.


The most common cause of flashing is a parasitic infestation. Parasites are a common problem in pond fish such as koi and goldfish and need to be eliminated quickly to avoid your fish getting sick and possibly dying. Primary parasite infections are common in ponds with new fish or live plants without proper quarantine. External parasites reside on the surface of the skin or within the gill filaments of fish. Internal parasites reside within the fish’s body, either within the digestive tract or the actual tissues. Most cases will require examining a scraping under a microscope to determine the exact type of parasite, but broad-spectrum treatments and/or the process of elimination may help you decide the best course of action.


Koi and goldfish are nearly always active in the moderate and warmer months. So a slow, sluggish fish in the spring, summer, or early fall could indicate a problem. A lethargic fish will tend to hang around a particular area, will move slower than usual, and might not indulge in its usual amount at mealtime.


Columnaris is an infection caused by bacteria that are always present in your pond. The hallmark symptom is fish hanging around the surface of the water and not swimming much. Fish typically only become infected when they are stressed and their immune systems are compromised. Fish that have recently been shipped are particularly vulnerable. Treatment with antibiotics should remedy the situation.

Internal Infection

An internal infection can cause sluggishness as the koi or goldfish’s organs begin to shut down. Unfortunately, it is often too late to treat once an internal infection is noticed, and treatments tend to be difficult and somewhat expensive.

High Nitrate Levels

Nitrates are the last chemical produced in the nitrification cycle. They are relatively not toxic compared to ammonia and nitrites, and in most ponds are consumed by algae. However, at levels above 80 ppm they can become dangerous, and your fish will start acting lethargic. Eventually, this could become a lethal problem if not resolved. Perform a water change, slow down on feeding for a while, and add plants like water hyacinth or water lettuce to help consume the nitrates.

Ready to add a few new fish to your pond? Shop our full selection of koi and goldfish.

1 response

  1. I bought a new koi it was about the same size as my goldfish. The goldfish that was in the pond were about the same size as they were chasing it biting it. the next day I found it dead. I took it back and I got another koi and put it in the pond and they were chasing and bite it. So my husband and I made a screen to divide it from the other fish and so far the other fish are just guarding the screen I don’t know what to do to get them to accept this koi.

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