A couple of years ago we looked at an overview of stress in Koi. Today, let’s take a deeper dive into the symptoms of stress, what may be causing them, and what you can do to alleviate the stress you may be seeing in your Koi.

Excessive Flashing

“Flashing” is the term used when Koi turn on their side and swim rapidly and erratically. The term probably comes from the bright flash of light that is often seen when the fish turns sideways and the sun reflects off the scales.

Possible Cause: Shipping Stress

Most Koi will be shipped at some point in their life, and it can be a stressful journey. Although shipping stress is typically easy for fish to recover from, they will exhibit some stress symptoms when they get out of the bag and into your pond or holding tank.  

Flashing is by far the most common symptom of shipping stress. When we release a bag of Koi from Hazorea Aquatics into our holding tanks, pretty much every fish in the bag will go darting around the tank rapidly. It is not unusual for Koi going from our holding tanks to your pond or tank to display the same behavior.

The good news is, this is almost always temporary. Koi are strong and hardy fish, and they typically recover very quickly from shipping stress. Typically in just 2-3 minutes after release, the flashing should subside.

Possible Cause: Parasites

If your Koi haven’t recently been shipped, are settled into your pond or tank, and are still flashing, scratching or scraping, it is likely a parasite issue. Parasites can happen in any pond, even if new fish have not been added lately. Snakes, frogs, turtles, leaves and all sorts of other “pond visitors” can carry parasites into the pond, setting the stage for an infection in your fish.

The optimal method for tackling a parasite issue is to “scrape and scope” your fish. This involves taking a gill clipping and slime coat sample, putting it on a glass slide, and examining it under a microscope to identify the parasite that has taken up residence on your Koi. Check out our article Putting Your Koi Under the Microscope for a more detailed look at scraping and scoping. After determining the issue, you can administer a targeted treatment that should clear it right up.

Unfortunately, many keepers are intimidated by the thought of scraping and scoping, and/or don’t have the equipment handy to safely perform this task. If you’re in this situation, and need to get your fish treated quickly, you can opt for a broad spectrum treatment such as Proform-C.  Proform-C will eliminate most (but not all) parasites that may be harming your fish. Without knowing what is on your fish, you can’t be certain that Proform-C will do the trick. But this option is better than nothing.

Excessive Scratching or Scraping

Scratching or scraping is similar to flashing, except instead of darting through the water, your fish are scratching their bodies against your pond wall or objects in your pond or tank.

Possible Cause: Parasites

Just like the flashing issue, excessive scratching or scraping on the sides of your pond, or objects in your pond, likely means that you have a parasite issue. Follow the same recommendations mentioned in the section on flashing to remedy your problem.


Possible Cause: “Excitement”

Ok, your Koi probably aren’t really “excited”, but we’ll go with that, for lack of a better term. If your Koi have just been moved to a new pond or tank, they are likely to jump for the first day or two as they settle into their new surroundings. This is normal behavior, and typically subsides within a couple days. At Next Day Koi, we keep covers on a new tank of fish for 2-3 days, then remove the covers and leave the top of the tank open afterward. Fish typically don’t jump after they are settled.  

It’s a good idea to be ready with a cover in case your Koi start to jump. Keep them covered for a few days, then take the cover off for a bit while you are around to monitor. If the jumping seems to have subsided, you’re probably safe to leave the cover off for good.

Possible Cause: Poor Water Quality

If your water parameters are way off, then it’s possible that the fish are jumping to look for an escape from the harmful conditions. Grab a test kit and run through a checklist of all the crucial water parameters. If something is really off (not just a little, but A LOT) then do what you can to get the parameters back in order as quickly as possible.

Possible Cause: Predators

If your fish are settled in and your water quality is on point, then it’s possible there is something chasing them around the pond and causing them to jump as a means of escape. Do your best to catch whatever it is out of your pond, then look for methods to secure your pond and prevent future entrance of predators.


Sluggishness is a bit of tough one. Koi are poikilothermic, meaning that their body temperature is dependent on the ambient temperature of their environment. As the water cools, their body temperature cools, metabolism slows, and they are less active. This type of “sluggishness” is normal and not cause for concern.

Possible Cause: Columnaris

Columnaris oh koi fish gillsWe’ve covered columnaris previously, but here’s a quick run down.

It is caused by bacteria that are always present in your pond. Fish typically only become infected when they are stressed and their immune systems are compromised. Fish that recently been shipped are particularly vulnerable, as their immune systems recover from the stress of shipping. Treatment with antibiotics should remedy the situation quickly.

Sluggishness and hanging around the surface of the water are one of the hallmark symptoms of columnaris infection. The key here is “hanging around the surface of the water”. If your fish is sluggish on the bottom, or sluggish in the middle of the water column, then it’s likely not columnaris. If your fish is hanging at the surface, just floating and not actively swimming, then it probably is columnaris. Check out this article on columnaris for more information and treatment options.

Possible Cause: Internal Infection

If your Koi is suffering from an internal infection, its organs are beginning to shut down and this can lead to sluggishness. Unfortunately, it is often too late to treat once an internal infection is noticed, and treatments tend to be difficult and somewhat expensive.

Gasping For Air

Possible Cause: Insufficient oxygen

If the oxygen level in your pond or tank is not sufficient for the number of Koi residing there, your Koi will likely hang at the top of the water with their mouth opening and closing above the surface of the water. We call this behavior “hurting,” but you may also see it referred to as “piping,” among other terms.

If your fish are hurting, it is imperative to either reduce the number of fish or increase the level of oxygen as quickly as possible. Increasing the amount of oxygen should be your first priority. Add an air stone or bubbler as quickly as possible to aerate the water and increase your oxygen level. If possible, spreading some of the fish to another pond or tank will also help, although this is not the ideal short term solution.

Possible Cause: Flukes

It’s possible that your oxygen levels are sufficient, but your koi are not getting the oxygen they need through their gills and into their bloodstream.  Most commonly this is caused by a fluke infestation on the gills.  

Flukes in large numbers can inhibit the function of a fish’s gills, preventing a sufficient amount of oxygen from being processed through the gills and into the bloodstream. There are several medications available with praziquantel as the active ingredient. Administered correctly, these should clear up a fluke problem.


Possible Cause: New Environment Stress

As we discussed previously, fish are excited and stressed when added to a new pond or tank. It is very normal for new fish in a pond or tank to seek out the smallest, tightest crevice or hole they can find, and hide there for a period of hours or even a couple days. This is normal, and the fish should come out and return to acting normally after it has become acclimated to its new surroundings.

Loss of Color

This is a tricky one, as it can be difficult to pinpoint what the cause is. Several factors can lead to the loss of color, including malnutrition, lack of sunlight and poor genetics.  

If you believe your Koi’s food may be to blame, make sure that you are feeding food formulated for Koi. Food for other species of fish may not contain the nutrients that Koi need to maintain their bright color.

If you are going to be keeping your Koi indoors for a long period of time, consider adding a UV light to your setup.  Long stretches indoors without supplemental light can sometimes lead to color loss.

Tattered Fins

Possible Cause: Bacterial Infection

A bacterial infection (commonly referred to as “fin rot”) can eat away at the tissue of your Koi’s fins, causing a tattered look.  If your fish are indeed suffering from fin rot, there will be signs of infection around the tattered area.

Unfortunately, there is no quick fix for fin rot.  The first step is to remedy any underlying stressors that compromised the Koi’s immune system and allowed the infection to set in.  There are some treatments available for bacterial infection, but they tend to be difficult and somewhat expensive.  Check out this article for more info.

Possible Cause: Predators

It’s possible that tattered fins are the result of a predator problem. If you’re seeing tattered tails, it’s quite possible that something is chasing your fish around the pond, snatching at them, but only getting a small mouthful of tail before the tail fin breaks and the fish darts off.

If you suspect that predators are an issue, you will need to remove anything that has gained entry to your pond, then work to secure the pond from further entry.

Pectoral Fins Clamped To Body

Possible Cause: Columnaris

Pectoral fins clamped tightly to the body are another hallmark of a columnaris infection. If you see this in even just one fish, move quickly to treat with antibiotics, as the infection is likely on the way for the rest of your fish. Check this article for more info on columnaris diagnosis and treatment.

Diagnosing and remedying the cause of stress in your Koi can be a frustrating endeavor. Be sure to stay calm and work through a process to eliminate possible stressors until you have arrived at the most likely issue. Keep in mind that even the most experienced keepers in the world are constantly learning and that any issues you work your way through will make you a more confident and knowledgeable Koi keeper for years to come!

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