How to stop Koi fish from flashing

traditional ornamental fish in pond, beautiful koi

The other day, I was at a friend’s place, admiring her pretty koi pond in the backyard. However, I observed that the koi seemed to be behaving a bit strangely- they were swimming swiftly and rubbing against the sides and bottom of the pond again and again. My friend was puzzled about this too, so I read up about it, and learnt that what the koi were doing was called flashing, something they do mostly when bothered by itching from parasites.

So, how can one stop koi fish from flashing? As there are many reasons for koi flashing, such as bad water quality, infection from parasites/viruses, temperature changes, etc, the best way to stop the flashing is checking and/or changing the water, and using the correct preventive measures or treatment.

Why do koi fish flash?

The most common and serious cause for koi fish flashing is irritation from parasites that can spread out in the pond and infect all the fish. Flukes, namely skin flukes and gill flukes are the most common parasites to infect koi fish by feeding on the skin and tissue of the koi, causing discomfort, itchiness and weakness to the koi over time. Flukes are mostly microscopic, so you’ll have to take a sample scrap of the koi’s skin or gills and look at them under a microscope. However, if the koi starts showing loss of appetite over time, and other factors such as water quality, sunlight, etc are stable, you can tell even without a microscope check that the koi is suffering from parasitic infection.

However, there are many other causes for koi flashing as well-

  • Adjusting to surroundings– Sometimes, when you bring in a new koi to your pond, it might need some time to adjust to the new living environment, the other fish in the pond, and the temperature and weather conditions. In such a situation, the slightly stressed koi might cope by showing the usual symptoms of flashing- swimming around fast or sideways and rubbing their bodies against the sides of the pond- but if this behavior stops in a few days, it’s definitely not flashing and there is no need to worry.
  • Nature of the koi– Just like we humans are rather temperamental and have our own personalities, some fish are different from others as well. You may find that a particular koi is exhibiting flashing behavior all the time, but if you compare with the other koi and find that they are not behaving the same way, it means that this is playful behavior for that koi and this is how its nature generally is.
  • Poor water quality– If you observe that more than one koi in the pond, or most koi are exhibiting symptoms of flashing, it is most likely a water quality problem. Other than most of the koi flashing, another way to identify this issue is if it occurs in a span of a few days. Water quality is known to shift and change very swiftly, so sudden koi flashing can be a result of that. A sudden spike in ammonia, for instance, in your pond water, can cause considerable discomfort and damage to the koi’s gills, causing flashing.
  • Viral infection– Apart from water quality issues, viral infections are the only other ones to show flashing symptoms in koi fish within a span of a few days. Viral infections also spread very quickly and most, if not all koi in the pond, will show sure signs of flashing because of this.
  • Change in water temperature– Water temperature for a koi pond is more important than it may seem. Temperature above 80° F reduces the solubility of oxygen in the    water, provides increased breeding ground for parasites, and is also known to cause significant distress in koi, leading to flashing. Temperature change can also occur  hand in hand with pH imbalance caused by heavy rainfall, which may result in reduced disease resistance in the koi, and increased flashing.           
  • Poor diet, sunburn (will mostly affect lighter colored koi if the sunlight exposure is too high), chemical or water runoff from other sources are also possible causes of koi flashing.

Ways to stop koi fish from flashing

Now that the various possible reasons for koi flashing have been discussed and examined in detail, it is time to move on to possible cures, prevention methods and how one can stop koi from flashing. It is important to take stock of all the factors affecting the koi’s overall health before making an attempt to treat it.

  • A good practice is to keep a regular check on the water condition of the pond. You will want to be particularly vigilant of the situation after a water change, chlorine treatment or if any other form of chemical treatment has been administered to the pond. One should try to test the water daily at a fixed time so that the temperature, pH level, ammonia etc can be checked and changes can be made accordingly to keep the koi comfortable. Be sure to dechlorinate any water you want to add to the pond, and it is a good idea to change 5-10% of the pond water every day. It is necessary to remember that pond water should never be changed more than 25% at once per week, lest it causes an imbalance in the water.
  • Since a dip in oxygen levels is often distressing to koi and causes severe flashing, a good idea is to provide a little extra aeration for the pond. Aerators of different sizes and varieties are easily available online and in stores as well, so you can keep more than one, such that each section of the pond is covered. A well-aerated pond is less at risk from parasites and dirt; it also keeps the oxygen levels steady so that the koi can breathe properly.
  • Whenever you introduce a new fish or plant to the pond, be sure to check on the koi as well as other pond inhabitants to see how they are adjusting. Some koi may be flashing because of the new pond mates or because of flukes brought in by the new entrants. It is important to watch out for that and act quickly in case it is fluke infection.
  • If you are certain that parasites have infested your pond and are causing koi flashing, there are certain chemical treatments available to get rid of the parasites and cure the koi. Aqua Prazi is a very safe treatment that can be administered to the pond, but if severe infestation is causing the flashing, you might have to consider stronger and harsher treatments like Chloramine. It is better to avoid the latter as much as possible because getting the dosage wrong can cause terrible harm to the koi.
  • If the koi is suffering from severe fluke infestation, a noticeable result of the same will often be the peeling off of scales and the development of small lesions or open ulcers on the koi’s body. In such a case, it is best to quarantine the affected koi in a holding tank and treat it accordingly for infection.
  • It is also a good idea to keep a lot of submerged plants, and some floating plants in the pond so that the koi can seek shelter from too much sunlight- otherwise they risk getting sunburn and feeling discomfort, which leads to flashing. Fish shelters and built in shades are also good options, but it is also important to ensure that about 40-50% of the surface is still left open to avoid stagnancy of water.
  • Koi tend to flash a lot when their skin and gills experience irritation due to chemical runoff from soil nearby. If you have a patch in your garden or yard that you tend to fertilize, it would be advisable to check the soil nutrient levels before doing so and absolutely avoiding phosphorous unless it’s necessary. Excess phosphorous will leak through the soil into the pond and will irritate the koi.

Related Questions

What kind of diet is recommended for koi fish?

While koi can eat almost anything, it is better to feed flakes to smaller koi and pebbles to big ones, carbohydrate rich food in autumn and spring and high protein in summers. Always invest in nutrient-rich food mixes. As a treat you can let koi have some human food like lettuce, rice, etc as well.

What other fish variety are good companions for koi?

Some suitable pond mates for koi are goldfish- they are great breeding partners for koi, red tail catfish, rainbow fish, barbel fish, and janitor fish which are bottom feeders and will not disturb the koi.

How can one prevent parasitic flukes from entering and infecting the koi pond?

Often, flukes enter the koi pond through predators, birds and their feathers, bird feces etc, so the best option is to set up fencing and protective screens to stop them from entering. It is good to keep an eye on the koi after letting new plants or fish into the pond, since they can also carry flukes.

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