Koi Diseases and Stress Symptoms

Koi Diseases

Let’s face it, there is no koi parent who doesn’t want to keep his or her fish disease-free. After all, it hurts us to see these beautiful creatures suffering. It also worries and frightens us knowing that whatever disease one fish may have contracted can spread swiftly to the others.

Instead of just one ailing fish, you soon have a pond full of them. Depending on the severity of the koi disease and the circumstances surrounding it, there’s the very real potential that you might lose your entire pond of beautiful, loving koi. It’s not fair, and it’s totally unnecessary.

No matter how big a pond you have, no matter how great your water quality is, if you don’t have healthy koi to enjoy, you don’t have anything, do you?

Koi Diseases and Stress Symptoms

Diseases of the Koi

There are various kinds of bacteria, fungi, parasites, and also other illnesses common to koi fish. Some of these koi diseases are common to other tropical pet fish. If you’re an expert on fish, then you might have heard of some of these common diseases.

If you are a newbie to the field of fish keeping, then these illnesses and diseases might be completely new to you. Anyway, it’s good to review the symptoms and get to know, so you can easily identify them in your pet fish.

Koi Diseases and Water Quality

At this point, I don’t think I need to convince you that disease and water quality (or lack thereof) go hand in hand. The following are the most important tests you can apply to help maintain your pond or aquarium’s quality of water. Good-quality water will usually help you to prevent the koi from generating new diseases.

It is to be ensured that the level of nitrate in the aquarium water should not exceed 50 ppm (parts per million).

You have to ensure that there is no ammonia and nitrite in your fish pond.

Avoid a population explosion in your pond. It is advisable to maintain a sensible fish load with a suitable stocking density in your pond.

Did you know: Actually Fish can grow faster in a pond environment where the density of stocking is lower? The bodies are normally reacting to broader areas of pond water.

The lower the ratio of stocking, the less amount of stress on the fish. You have decreased the competition amongst the koi fish during the period of their feeding. (I bet you didn’t even realize they were competing!)

Koi Diseases and Stress Symptoms

When a Koi fish first recognizes it is under stress, its body releases adrenaline, the same way we get a rush of adrenalin when we experience stress. The hormone gives the koi the strength and stamina it needs to make a quick getaway.

How Stress Affects Koi Fish

The release of this adrenalin is absolutely essential to health and when predators are present, the life of your koi. But it doesn’t occur without consequences.

For starters, the release of adrenalin can seriously and adversely affect the balance of salt and water in the pond ecosystem. And this in turn can seriously affect your fish’s immune system.

Remember that mucus your koi produces? Its role is to create a physical barrier between the environment and your koi. It also establishes a barrier between the fish and parasites.

When the water conditions deteriorate, your fish produces more mucus. Adrenalin produces poor water conditions, which in turn, cause your fish to produce even more mucus.

This excessive mucus encourages the growth of parasites. In fact, two parasites in particular just love to munch on mucus: the Costia and the Trichodina.

Koi Stress Symptom – Induced Cortisol

But wait, I’m not quite finished yet! Let’s go a step beyond this. The presence of stress also produces another hormone, cortisol. It’s the same hormone we produce when we react negatively to a stressor in our lives.

Cortisol compromises the actions of your fish’s immune system. Specifically, it hampers the ability of your fish to manufacture white blood cells, which are crucial to the health of its immune system.

This increases its vulnerability to koi disease and illness. It’s a double whammy of potential ailments!

In an ideal pond world, the goal is to remove as many stressors as possible, as quickly as possible. This would mean eliminating the ammonia and nitrite in the water and improving the oxygen levels.

Of course, not all stressors come in the form of bad water. A bird or a cat might be lurking around the pond. The cat might have a fork and knife in its hand, waiting for just the right moment to grab dinner….

This would stress any fish out. If this is the case, eliminate the cat from the equation. (He’d be easy to identify; he’s the only cat wearing a napkin around his neck!)

Avoid and Eliminate Koi Stress Symptoms:

Once you’ve pinpointed and eliminated the cause of tension from the environment, be patient. It may take time for your fish’s stress levels to return to normal. Until that occurs, your fish is still vulnerable to illness.

I’m about to break this down one step further for you. First, I’m going to talk briefly about the prevention of koi diseases. Then, we’ll talk about how to maintain the health of your koi.

This may sound like I’m splitting hairs (er, gills?), but the truth of the matter is, it’s much easier for us to treat these as two distinct topics. In real life, the areas overlap. But you’ll be able to better understand the problems when they’re presented like this.


Dibyajyoti Bordoloi is the creator and author of FishCampRehab.com, a third-generation experienced fish keeper and owner of a successful pet breeding farm. He is also a member of the Center for Wildlife Rehabilitation And Conservation (Assam), the Marine Aquarium Societies of North East India, and the Kaziranga Nature Conservancy of Assam.