What Would Cause Koi Fish to Die?

What Would Cause Koi Fish to Die?


Many people keep koi fish in their outdoor ponds. Why? Because they are a beautiful addition and quite cute. Even though koi are resilient and can live for quite some time, that doesn’t mean they’re invincible. Knowing when your koi fish is not feeling good is one of the most crucial aspects of Koi care. Since there are a variety of factors that can lead to a koi fish’s death, it’s important to know what would cause koi fish to die.

Poor water quality is the leading cause of koi fish death. Your fish’s lifespan is directly proportional to the standard of the water they are kept in.  Having enough fish in the pond, performing frequent tests, and having a good filtration system are all important parts of successful koi fish care.

Below I explore the most common causes of death. keep reading!

Parasite infestation

Koi fish can die from parasite infestation. Koi, like all other forms of life, are susceptible to parasitic infestation.  Parasites are an issue for fish, but you won’t know it until your fish starts showing symptoms, at which point the infection is probably fairly severe.  Fortunately, most parasitic infections respond well to a carefully regulated regimen of anti-parasitic medicine introduced into the water.

The secret to preventing the loss of koi to parasites is keeping an eye out for fish that aren’t thriving.  Koi that stop eating, seem listless, or have lost their usual vivacity of color may have parasites.

Using gravel or rocks as pond liner

During the colder months, koi will spend more time resting on the pond’s floor, exposing their delicate bellies to the substrate there.  If your pond is lined with gravel or rocks, your koi are at risk for harm and death from ulcers and infections.  Many koi keepers specifically seek out ponds with a smooth bottom for this purpose. The simplest way to prevent this problem is to use something other than gravel or boulders as a pond liner.

Susceptible to predators

The neighborhood dog and the seemingly cute cat are some of the predators that enjoy the tasty meat of koi.  You shouldn’t expect them to appreciate your pond’s beauty.  They’ve found an easy dinner in your pond, so they’ve come to the feast.  Preventing harm from predators to your fish can be a full-time task, but it’s well worth it.  A bird net and a pretty fence can often deter most predators.

Not enough oxygen

Even seasoned koi lovers can fall victim to this rookie mistake if they aren’t monitoring the development of their fish carefully enough.  Competition for resources is the most frequent cause of low oxygen levels in pond water.  When designing a pond, it can be challenging to strike a balance between the amount of fish, plants, and supplemental oxygen sources.  The fact that this is a dynamic crisis only makes matters worse.

What is the reason? It is because oxygen consumption rises in tandem with a koi’s size.  In the spring, a pond, for instance, can be maintained in equilibrium with six-inch fish.  These fish can double in size as the season advances, putting a strain on limited resources.  If more oxygen isn’t introduced, or some of the fish aren’t taken out, the entire tank could die from lack of oxygen.  You can avoid this problem if you think about how many larger fish your pond can handle and stock it accordingly.

Jumping out of a pond or container

Most experienced koi keepers have dealt with their share of suicidal fish by this point, so this problem primarily impacts newer keepers.  Koi are notorious jumpers, whether they are chasing each other, chasing a meal, or are just unhappy with their present surroundings.

Those who have attempted to care for koi in a tank without a cover will attest to this.  Most fish will be successful in their attempt at a vertical takeoff and will splash back down into the water, but occasionally one will not.  It’s nearly impossible to stop every fatality caused by an overzealous acrobat in your pond unless you have someone watching it constantly.  Consider putting up a bird net if you ever have a problem with a jumper.


Koi can get sick from viruses and bacteria just like any other fish, but if they’re healthy they usually have a decent chance of beating the infection.  You can do a lot to keep your koi healthy and clear of disease by keeping the water clean, removing any sharp rocks, and feeding them a varied diet.

Presence of pesticides

If you’re like other responsible pond keepers, you won’t put harmful chemicals into the water.  Nonetheless, runoff from torrential rains can make its way into your pond, drowning your fish.

The amount of runoff that your pond will be exposed to and the money that you will spend to restock your pond every few months can be greatly reduced with careful planning during the pond’s building phase.  However, if you discover that your pond is negatively impacted by runoff after a heavy storm, you should create a drainage system to reroute the water elsewhere.


Overcrowding a pond with too many Koi is the most typical mistake made by Koi keepers. It’s true that less is more when it comes to Koi. Too many Koi in the tank could be to blame for their untimely demise. As the population of Koi grows, there may be more fish vying for food and shelter.

Koi require more care as they grow in size. To give just one example, imagine a tank with 50 Koi in it; not only do they all need to fight for room and food but there will also be a lot of waste to deal with and stress from being in such a small area. As a consequence, oxygen is depleted and death sets in.

Use of algaecides

Algae blooms are a common occurrence in a Koi pond. In addition to providing a tasty snack for the Koi, these algae may also help maintain a healthy nutrient balance in the water.

Scrubbing the surface and adding some algaecides will remove the soft green algal. Increasing the dosage of algaecides will do more damage than good and could even cause the fish to die out of nowhere.


It’s scary to think that your koi fish would die, particularly if you have no idea what would cause its death.  is not always possible. If you investigate the aforementioned causes and take corrective action, you may be able to save and revive your Koi from a similar fate.

Here are some other related articles:

Do Koi Fish Hibernate?

How Many Types Of Koi Fish Are There?

Can Koi Fish Live With Goldfish?