Koi carp, cousins of the resilient common carp (Cyprinus carpio), are quite hardy by nature due to both evolution and selective breeding. However, just as with any organism, they are still sensitive to certain environmental stressors and irritants.
The most common symptom of these is flashing, which consists primarily of jumping and dashing around the pond, rubbing their scales on rocks and the pond’s edges, and swimming on their sides.
These erratic behaviors are oftentimes classic indicators that something is not quite right in your pond, and you should begin assessing your pond and koi immediately!
When is Koi Flashing a Concern?
Flashing behaviors can occur with new fish as they grow accustomed to their new environment. They’ll likely be stressed for a time as they acclimate, but in this case flashing and similar unusual actions should only last for a couple of days.
If flashing occurs consistently and persists for more than a couple of days, it may be a sign of health issues, skin irritation, and underlying problems related to illness and/or water quality issues. If only one or two fish are flashing and it doesn’t occur consistently, it may just simply be something that they do!
For example, some fish will flash, dash, and jump about after eating simply because that’s their personality. However, if multiple fish, particularly if it’s most of all of the pond, there is likely an extensive infection that is being passed about, water quality could be poor, there could be too much direct sunlight, etcetera. The video below shows flashing/rubbing behaviour and what to look out for:
What Are the Major Causes of Koi Flashing?
1) Koi Flashing After Food
Sometimes, the gill rakers (located on the gill arches, and functioning as a food filtering system as the koi eats) become clogged with food when eating. This can result in koi flashing behavior, as they work to free their gills of this excess food. You can see this more clearly in the video here! Koi flashing after eating is actually one of the most common reasons many koi flash, and could be an indicator that the food you’re using is not suitable to the fish or pond.
2) Poor Water Quality
Poor water quality includes things such as pH and KH that are too high or low, too little dissolved oxygen, the presence of nitrates and ammonia, too much organic matter such as grass clippings and fallen leaves in your pond, algae that visibly clouds the water, lack of plants and filtration, and high salinity. All of these create an unstable environment that can result in fish stress, illness, and disease.
Even water temperature (between 59 and 77° F is best for koi) is an issue, as water that is 77° F is able to hold 8 parts per million of dissolved oxygen – above 80° F, and this drops down substantially to 6 parts per million, as the solubility of oxygen gas decreases as temperature increases. Not only does this warm, anoxic water create prime habitat for bacteria and parasites to establish, but water that falls below 6 ppm of dissolved oxygen is considered stressful and harmful to fish, with 7 ppm, or preferably above, being best.
3) Parasites & Sickness
Parasitic, fungal, bacterial, and viral infections quite often result in flashing behavior, as koi try to alleviate some of their discomfort and pain. Gill and skin flukes, ich, and koi herpes virus result in itching, gill and scale irritation, pain, potential fin rot, scales falling off, and even mortality if not properly treated, with flashing being a telltale sign of their presence.
If these afflictions are not taken care of immediately, not only can they result in fish death, but they can spread to outside waterways and infect wild fish as well. For instance, many studies over the past decade have found that koi herpes virus is able to spread to wild common carp in nearby waterways, making it of paramount importance that infected koi be immediately quarantined and properly treated.
4) Heavy Rainfall
Heavy rainfall alters pH, and is also quite high in nutrients due to the nature of the water cycle. Such a large amount of water falling into your pond can also potentially alter the water temperature quickly enough that your fish become stressed and resistance to illness could become reduced. As a result of the pH change, nutrient overload, and possible temperature fluctuation, koi may begin flashing due to stress and discomfort.
5) Sunburn Due to Overexposure
Koi, particularly lighter colored ones, are prone to sunburn just as we are. If your koi has sunburn, you’ll notice a slight pinkish hue that becomes darker and angrier looking the worse that it gets. If it’s irritating their skin, flashing and jumping about is a likely response as they try to cool the area and soothe their skin.
6) Faulty Electrical Equipment
Sometimes, electrical equipment such as lights, fountains, pumps, and any other equipment in contact with the water can leak voltage. It can occur in a small enough amount that it doesn’t trip the breaker, but the voltage can irritate fish that swim near the equipment and cause them to flash and behave in other irregular ways. You can check for leaked voltage simply by using an electrical meter to measure currents in your pond.
7) Large Water Changes without Dechlorinating
Water changes should ideally occur in small amounts frequently – changing 5 to 10% of the water daily, or 15 to 25% if done weekly. The larger the water change, the more of a potential shock to your koi. This is particularly true if the water is not being properly dechlorinated before being added to your pond – not only will the water change itself stress your fish, but any chlorine present will harm them and can cause chemical burns, resulting in flashing behavior.
8) Poor Diet & Nutrition
Flashing behavior in koi, particularly staying toward the surface of the pond floating on their side, can be caused by poor diet. All koi should be fed a varied diet that can include spirulina, wheat germ, citrus fruits, raspberries, shelled peas, bloodworms, and a variety of other supplemental treats. Well-balanced pellets or wafers made specifically for koi should also be fed, whether they’re store-bought or homemade.
9) Chemical and Nutrient Runoff
Any chemicals applied to your lawn or garden, such as insecticides, are likely to run off into your pond with the next rain. These obviously negatively impact water quality, and can cause all manner of health issues in your koi. Similarly, fertilizers and grass clippings are also likely to wind up in your pond, and can lead to nutrient overloads that create a perfect breeding ground for bacteria, fungi, and algae that can then result in illnesses such as ich developing.
How to Treat Flashing & Prevent Flashing (Step-by-Step)
Whatever the cause of koi flashing, there are ways to remedy it and prevent it from occurring again.
1) Test & Improve Water Quality
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Water quality should be tested daily, preferably at the same time each day to ensure accurate results. You could even test multiple times per day, if you strongly suspect that water quality is the culprit and want to have readings that reflect variations that occur throughout the day as lighting and temperature are likely to shift.
Make sure that you are treating your pond as needed to maintain proper pH, hardness, and nutrient levels while minimizing the presence of harmful compounds like ammonia and nitrates.
2) Provide Extra Aeration
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This one goes hand-in-hand with water quality. Extra aeration will keep water moving, lessening the likelihood of algae, parasites, and bacteria being able to establish and reproduce, and will also help to mix water so that temperatures and oxygen levels remain constant throughout. Aeration can be achieved with the use of electric air pumps, water falls or fountains.
3) Treat for Parasites (just in case!)
- Aqua Prazi
- No temperature restrictions
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Treating your pond for parasites will, of course, rid the water and your fish of most parasites. Aqua Prazi in particular is quite safe to use if parasites are the suspected culprit, and very effective for killing off parasitic flukes and worms. Stronger treatments, such as Chloramine-T, are very effective but potent and reactive, requiring exact dosing. Therefore, brawnier chemicals like Chloramine should only be used if parasites are for sure present, and as a last resort as the wrong dosage or even certain water parameters can render the treatment toxic to your fish.
4) Check Pond Equipment
- Multimeter measures up to 1000V AC/DC voltage, 10A AC/DC current and 40MOhms resistance plus temperature, capacitance, frequency,...
- Digital Mulimeter has CAT IV 600V / CAT III 1000V safety rating
- Built to endure a 6.6-foot (2 m) drop and withstand daily wear and tear on the job site
Pond equipment should be checked regularly to ensure proper functioning, as well as to safeguard against potential voltage leaks into the water. If you suspect voltage leaking to be an issue, you can test for this using a voltage meter.
5) Water Changes & Always Dechlorinate Water
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As mentioned above, smaller water changes conducted daily or weekly are less likely to stress your fish. If you can, change out 5 to 10% of the water daily, or no more than 25% weekly.
Also, chlorine is incredibly harmful to koi (and all fish), potentially resulting in chemical burns and removing the protective slime layer from their scales. Without this slime layer, koi are much more susceptible to infections and disease. Always make sure to dechlorinate any water added to your pond; this can be done easily with a carbon-filter hose attachment (pictured!).
6) Provide More Shelter & Shade
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To protect your fish from too much sunlight, incorporate fish shelters and plenty of submerged, floating, and emergent plants for them to seek sanctuary in. Keep 25 to 50% of the pond surface uncovered, though, to prevent stagnant conditions from developing. Plants in your pond will also help to filter the water and keeping the temperature more cool and constant. Similarly, you can plant trees near your pond or install shade sails to provide shade.
7) Feed Only High Quality Foods
Your koi should only be fed foods that contain high quality ingredients from trusted sources, such as those we recommend here! Avoid cheap pellets, as these likely don’t fulfill all of the nutrient requirements of koi carp, which are much the same as those of wild common carp.
8) Avoid Using Fertilizers and Chemicals on Your Lawn/Garden
If at all possible, do not use fertilizers or chemicals on your lawn or garden as these can wash into your pond. If you must use fertilizer, please test your soil first to see which nutrients are actually needed – that way you’re not adding phosphorus to soil that already has plenty of it, which will then leach directly into your pond as the soil and plants will not be able to make use of the excess.
What would cause koi to be cautious about eating?
Hi Mac,
Has this koi always displayed issues with eating, or is this a new development? Do any other symptoms accompany this lack of wanting to eat, such as flashing, erratic behaviour, any skin lesions, etc.?
It could be a number of things: stress, simply shyness if it’s a new fish or something has recently changed in the pond/aquarium, some form of infection (either from bacteria, a virus, fungi, or parasites), or the fish simply doesn’t like the food or the food doesn’t agree with it. I know that’s all rather vague! Let us know if you notice any other symptoms and perhaps we can hone in on it more. In the meantime, try offering a variety of foods to see if the koi are more willing to eat some more than others, or aren’t interested in any at all.