Koi Fish Fins

Koi Fish anatomy:  A tale of the tail

One of the most beautiful features of Japanese koi fish is their fins and tails. With a flowy elegance, koi fish fins and koi fish tails can be mesmerizing to watch. Second only to koi fish whiskers, fins are probably the most distinguishing characteristic of koi.

We'll start by taking a look at the anatomy of the koi carp and discuss issues that affect the tail and fins.  Koi have the same general anatomy as other freshwater fish.

Group of Koi with Different Colors
A Beautiful Variety of Koi With Mesmerizing Fins

Koi Fish Fins and Shape

Koi fish have three pairs of pectoral fins located behind each side of the body. Each pair contains one long ray and several shorter ones. In addition to these, there are four pelvic fins located between the anal opening and the caudal peduncle.

There are no gills, so koi rely solely on their swim bladder to breathe. This allows them to live underwater without having to come out every few minutes to take a breath.

The dorsal fin runs along your koi's spine from head to tail. It has two parts: The first part consists of soft rays that are rounded off like feathers; they help with stability when swimming. The second part is made up of hard sharpened spikes called denticles. These serve an important purpose by helping to keep the koi stable while moving forward.

A koi fish's eyes are set into the top of its head near the center line. Their pupils are vertical slits, allowing light to enter but not water to escape.

Photo of two koi with black markings
Observe Your Koi's Fins and How They Swim To Identify if They Are Healthy

Koi eyes are very sensitive to movement and change color depending on what direction you're looking. If you see something move quickly across your view, chances are it was a koi trying to catch prey!

A koi fish's mouth is located just below the eye socket. Its teeth are small and pointed, designed to grab food items such as insects and worms. Unlike humans, koi do not chew their food before swallowing. Instead, they swallow whole.

Anatomy of the Koi Fish Tail

Your koi fish's tail fin is composed of five sections: the base, the middle section, the upper portion, the lower portion, and the tip.


This is where the entire length of the tail fin begins. It is usually about half the size of the rest of the tail fin.

Middle Section

It starts right above the base and extends all the way down to the end of the tail fin.  It is typically twice as wide as the base.

Two Different types of Koi
Healthy Koi Should not Have Inflamed and Red Patches on Its Fins

Upper Portion

Right above the middle section lies the upper portion of the tail fin. This area is wider than the middle section.

Lower Portion

Below the middle section lies the lower portion of the tail fin, which is narrower than the middle section. At the bottom of this area is the point of attachment to the body.


At the end of the tail is the tip. It is shaped like a wedge and is often colored differently than the rest of the tail. Some varieties may even have spots or stripes.

Koi Disease Diagnosis and Treatment

Since we are on the topic koi fish anatomy, discussing common diseases will be helpful to understand what you and your koi may experience and what you can do to resolve or help prevent any problems.  Typically, you may notice something not normal with the koi fish tail or one or more of the other fins.

A Koi with White Base with Orange and Black Markings
Exercise Good Management Practices To Keep Your Koi Looking This Healthy

There are many different types of illnesses that affect both domesticated and wild-caught koi fish. Although some of these ailments are more prevalent among certain breeds, others occur throughout the species. Here we’ll discuss how to diagnose and treat some of the most commonly seen problems.

Koi Fish Fins - Tail Rot

If you notice that your koi fish tail or koi fish fins are not looking right, that is usually the first place a disease is noticed, but could be related to another issue. 

Koi fish tail rot is a secondary result typically related to another problem.  The primary issue could be poor water quality or water temperature or perhaps some stress factor on your fish.  Your pond filters may not be working well.  You may have too many fish in the koi pond.  The oxygen level of the water is low.  The nutrition could be poor.  The primary issue weakens the koi and allows bacteria to do harm to the koi fish tail and fins.

Treating Koi Fish Tail Rot

As a start, medication is available to help your koi fish tail or fins recover from the rot.  General medications available are safe for the koi and will not impact the water quality in your habitat. 

If your koi has a case of tail rot that is more progressive and has spread closer to the base of the tail or fins, an antibiotic food treatment may be needed.  If antibiotics are needed, use care with the administration of that plan as that can cause water quality issues and promote bacteria growth that can become resistant to the medication. 

A Sample Photo of a Fish With Bumps on its Scales
Check Your Koi for Bumps on Their Scales

The water will need to be tested and your attention should be focused on the pH, ammonia, and nitrates.  If any one of those is out of balance, it can cause trouble.  A more aggressive treatment may be needed if a general treatment does not solve the problem.

Koi fish tend to be larger than other aquarium fish and trying to quarantine them from others in your pond or habitat is not always an option.  However, rot on koi fish fins is not as likely to spread if you treat the source cause.  Separating a sick koi will not solve your problem if the problem is with the habitat environment.   

Ultimately, the best course of action is to treat your koi while at the same time determine the source cause and remedy it.

Do the koi fish fins grow back?  Yes, they do!  After treatment, the fins should return to their former healthy shape and color as long as no other issues are found.  In most cases koi fish tail or fin rot will not progress past the base of the fin.  In some cases, it may progress further, and those cases should be directed to a professional to evaluate.

Other Common Koi Fish Diseases and Treatments

White Spot Disease:  White spot disease occurs due to stress caused by overcrowding or improper feeding practices. When the population density gets too high, bacteria multiply rapidly causing unsanitary conditions for the fish. Over time, the skin becomes irritated and infected leaving ugly blotches.

White Spot Disease Treatment: To prevent future outbreaks, make sure your koi fish receive enough space per individual. Also, avoid overfeeding because excess nutrients encourage algae blooms. Keep the water clean and free of debris.

Platinum Ogon Koi in a Pond
To Maintain Healthy Fish Like This Koi, You should Avoid Overcrowding and Overfeeding Them

Ich:  Infected fish become lethargic and lose appetite. They also stop eating altogether. As the infection spreads through the body, symptoms include loss of balance, difficulty breathing, and eventually death.

Ich Treatment: Remove any dead fish immediately and quarantine the remaining healthy individuals. Clean the tank thoroughly using hot water and bleach solution. Afterward, add fresh water and test pH levels daily until optimal values are reached. Use an anti-bacterial medication if necessary. For additional information visit our article here.

Bloat:  When a koi experiences bloat, his/her stomach swells up making him/her look bloated. The condition affects mostly juvenile koi when they eat large amounts of food. Bloating causes them to feel uncomfortable and weak.

Bloat Treatments: Place the affected fish in a separate aquarium and feed only once every 2 days. Do not force feed. You should never put anything else inside your koi fish’s mouth. Feed live foods such as brine shrimp, bloodworms, daphnia, and mosquito larvae. Avoid giving frozen foods since they contain ice crystals that damage internal organs.

A Photo of a Koi With Dropsy
Isolate and Quarantine Sick Koi

Lethargy:  If your koi fish has been acting sluggish lately, he/she might actually be sick. Lethargy is characterized by listlessness, slow movements, lack of interest in normal activities, and decreased activity level. In severe cases, koi develop ulcers on their bodies that lead to bleeding.  If you notice any signs of illness, consult your veterinarian right away.

Lethargy Treatment: First, check whether there is any injury or wound present on the fish. Next, remove any foreign objects from the fish's mouth and examine the eyes carefully. Then, place the fish back into the original container where it was kept before being removed. Finally, keep the fish warm while waiting for veterinary assistance.

A Discolored Koi Near The Edge of the Pond
Observe Your Koi If They are Discolored and Keep Scraping On To The Sides of Your Pond

Scales:  Although scales are considered one of the best features of a beautiful koi fish, they can sometimes get stuck together resulting in small patches of missing scales. This problem happens mainly during winter months when cold temperatures reduce metabolism. However, it can happen anytime. Scaling is easily treated but requires patience. Simply use fine emery paper to gently pry off each scale individually. Once all of the scales are gone, apply a medicated ointment to help heal the damaged tissue.

Outlining some of the issues that can occur provides examples of how the prevention of problems is always less effort than the time and effort to fix them.  Some simple steps in tending to your pond water, will go a long way to keeping conditions optimal for your koi, especially those beautiful koi fish fins!

Frank Salvatore

In 2015 our family moved into a house south of Denver, Colorado with a  koi pond.  Since that time I've learned to really enjoy the koi fish and the pond. This blog is dedicated to providing helpful hints and information for koi pond hobbyists - as well as those of you who just inherited a koi pond and are thinking NOW WHAT?

About Me

Hey there - I'm Frank Salvatore. In 2015 our family moved into a house south of Denver, Colorado that had a Koi pond. The problem was I knew absolutely NOTHING about koi ponds.

This blog is dedicated to providing helpful hints and information for koi pond hobbyists - as well as those of you who just inherited a koi pond and are thinking NOW WHAT?
Learn More About Me

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