Why are my Koi Losing Color? (And How to Fix it)

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many koi in a pond
Koi are known for their beautiful and diverse colors, and a loss of color could be a sign of a problem that needs investigating. Public domain.

One of the most spectacular features of ornamental koi, and also the main thing that separates them from regular wild carp, is their fantastically broad color pallet. Unlike other species of freshwater pond fish, koi not only come in a range of shapes and sizes, but also countless color patterns, and it is these unique colors that draw most people to the hobby in the first place.

The main goal of koi keeping is making sure your koi are happy and healthy, which in turn ensures your koi maintain a strong and vibrant color throughout their life. Sadly, sometimes things can happen which can cause koi to start losing their colour, which gradually dulls over time. A koi that is changing in color is often an indicator of a problem within its environment they’re unhappy with which is causing them stress or discomfort.

Common Causes for Koi Color Loss:

Although koi often start losing color due to the environmental problems above, sometimes it’s just natural and there isn’t anything you can fix. For example, koi will naturally dull in color as they age, so older koi will look much less vibrant in comparison to young and growing fish still developing their color. Likewise, during spawning (breeding) a female koi may lose some color due to the stress of the process, but this is often temporary and returns afterward so long as this fish is still healthy.

If your koi aren’t old or breeding, or the change in color is quite rapid, it’s best to take steps to try to determine the cause of the color loss before symptoms get worse. Keeping koi happy is our main job as fish keepers; and the happier our koi are, the healthier their color will be!

Do Young Koi Change Color As They Grow?

why do koi change color
Young koi often look extra colorful, but their pattern can change completely as they grow. Public domain.

Yes! Many people are attracted to young koi as they often look extra vibrant and colourful, but the color you see is by no means a lifetime guarantee. As koi grow they’ll change color and pattern, and some may go from patchy all over to totally one tone (often white) after a few years. This is normal and not something you can control, although you can sometimes make better predictions by looking at the fish’s bloodlines before purchase.

Only after 3-4 years of healthy growth will koi retain their mature color and pallet, so if you have a very young koi, the change you’re seeing could just be genetic. We always recommend purchasing older koi if you’re picky about the color you want, as young koi can sometimes be a real gamble. If you want to take a chance, paying very close attention to the bloodlines of your fish is important, as a koi from a weaker gene pool is much more likely to fade in color.

Luckily, however, there are still things you can do to enhance a young koi’s color as it grows, and if you get lucky with genetics, the end result will be even more spectacular!

Are Koi More Colorful In Colder Water?

Another yes! If you’re American and have visited any larger koi shows, you’ll notice most are held in summer. However, the biggest and most important Japanese koi shows are always held in Winter, and there is a good reason for this – colder water enhances color!

A cooler water temperature brings out a much more vibrant color in koi;  intensifying whites, reds, and deepening blacks. The lower water temperature means koi eat less due to a slower metabolism, which brings more color to the surface. This rule holds true for koi such as Showa, Sanke, Shio Utsuri, Sumi, and many other varieties of fish. In the summer, most koi will fade in color, so if you notice your fish look slightly duller than a few months before, it could just be the change in weather and temperature.

How to Improve the Color of Koi

1) High Quality Food & Nutrition

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One of the most common reasons koi may be losing their color is due to poor quality food and nutrition. In fact, 9 times out of 10 a koi purchased from Japan (or elsewhere) that is changing in color is due to poor diet and/or conditions in their new home. If food and environment is kept to a similar high standard, there is a much higher chance the koi will retain their color into adulthood.

If your koi are still young (less than 5 years), the food you provide should be rich in high-quality fats, protein, and extra vitamins. The fats will help the koi put on weight and make winter easier, and the protein will accelerate muscle development. Adult koi also need fats, but will be fine on a lower content diet so long as protein is within a range of 30-40% and comes primarily from an aquatic source. A problem with “budget” koi food is they’re not just low on protein and other essential nutrients, such as vitamins, their protein is often planted based, which isn’t ideal for growing koi.

As well as this, poor quality feeds will have more ash content, which acts as a filler that passes through the koi and contributes to waste in your pond. Selecting a koi food with a decent protein source (fish/shrimp meal), and good range of fats (4-10%), and low ash content (<10%) is recommended for the best growth and development.

High-quality koi feeds which are designed specifically with koi in mind will also contain a wide vitamin and mineral profile, and often come with natural or synthetic color enhancers. Vitamins will help build a strong immune system, and color enhancers make the pattern of your koi much more vibrant.

For more information on the best feeds, check our full guide on koi food which includes feed recommendations and some useful nutrition guidelines. We also have in-depth analyses of individual koi food brands and types.

2) Test & Improve Water Quality

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Even if your water looks clear, this doesn’t necessarily mean you have good water quality. A gradual change in substances may not present as a change in clarity, so it’s always good to test water if you notice your fish are losing their color or acting strangely. Stress from increasing ammonia, pH, and changes in KH are all common causes of a loss in vibrancy and change in behaviour in koi. These kinds of problems with water quality tend to occur in early Spring and late Summer, so regular testing throughout the year is good practice if you have a lot of koi in your pond.

If water shows no signs of change, which is often the case, you’ll need to be on the lookout for changes in the behaviour of your fish. If your koi are unhappy with their conditions, they’ll certainly try to let you know – you just need to notice it! Koi that are constantly hiding, not taking food, or swimming in circles, all present behaviour common in poor water quality conditions.

Water test kits can provide a wide range of measurements, and should help you determine if the color loss is due to water quality. If the problem is a rise in ammonia or nitrites (often the case), better filtration, more beneficial bacteria, and manually removing sludge may be necessary. pH should never be changed rapidly, as this is much more dangerous to koi compared to a high/low stable pH reading. Often pH will resolve naturally over time once other problems, such as ammonia/waste build-up, are back in check. For more information on testing water quality and what measurements to look out for, check our full guide on water testing here.

3) Optimize Filtration & Aeration

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Something which will help improve water quality and overall water condition is optimizing filtration. A poor quality filtration process could be the cause of ammonia and waste build-up that is placing extra stress on koi. This could be due to a filter box that is too small for your pond, damaged filter media, or lack of beneficial bacteria for bio-filtration. In terms of koi keeping, water filtration should be one of our main priorities, as koi will either thrive or suffer depending on the filtration you have in place.

If your filter box is the correct size for your bio-load (waste levels), changing your mechanical and biological media may be a good choice if you feel it’s underperforming – a rise in ammonia being a key indicator here. Filtration can also be supplemented with beneficial bacteria products or activated carbon, which will help remove heavier organic pollutants that normal media cannot remove. Regular cleaning may also be required if you have a lot of debris build-up, with both mechanical and bio-media suffering when clogged with “gunk.”

As well as good filtration, koi will need plenty of oxygen in the water to main their color and stay healthy. Low oxygen conditions will cause all sorts of problems, with a dulling of color being just one of many. If you don’t have any natural aerators, such as fountains or waterfalls, adding a dedicated air pump to your koi pond is recommended for maintaining stable oxygen content. This can also be run all winter alongside a heater to ensure koi have the best hibernation/torpor possible, keeping their strong colour into Spring.

4) Reduce All Causes of Stress

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A stressed koi will begin to dull over time, so removing as many possible stress sources as possible is important for maintaining good color. Common causes of stress include changes in water quality, predators, lack of shelter, and the introduction to new environments. Koi are quite sensitive to changes, and much prefer a comfortable and stable living space. New koi introduced to new ponds, or koi during a heavy water change, may become stressed for a few days (or weeks) while things settle down. The arrival of predators to your pond can also cause issues with stress, as fish become more cautious about approaching the surface in case of an attack. Implementing predator deterrents, and providing either natural or man-made shelters is good practice to avoid this kind of problem.

Water should always be cycled in new ponds before adding fish, and it can also be de-chlorinated during heavy water changes. During times of high stress, tonics can be supplemented which contain natural ingredients to calm koi, but this won’t stop predators or fix poor quality water, so we only recommend this after the root cause is solved. Afterwards, however, stress relief products can provide a boost to koi which are feeling under the weather, helping dechlorinate water, remove heavy metals, improve their coats, and heal wounds and infections – all of which help reduce stress.

Likewise, if your koi is suffering from an injury, infection, or parasite, you’ll see both a change in behaviour and a change in appearance. Sometimes injuries can heal on their own, but they’re best treated as they may often lead to infection or become a breeding ground for parasites. Treating both bacterial infections and parasites is common practice for koi keeping, and should be done at the first sign of symptoms. It is good practice to treat for both problems just before winter (hibernation) and at the start of spring for the best possible start to the year.

Chris G
About the author

Chris G

Pond consultant and long-time hobbyist who enjoys writing in his spare time and sharing knowledge with other passionate pond owners. Experienced with pond installation, fish stocking, water quality testing, algae control and the troubleshooting of day-to-day pond related problems.

Read more about Pond Informer.

10 thoughts on “Why are my Koi Losing Color? (And How to Fix it)”

  1. I started noticing discoloration in my koi few days after I applied aqua tint in to the pond.
    Can the aquatint cause discoloration of the koi fish?

    • Hi Salau,

      Thanks for the comment!

      Sadly, I’m not familiar with Aqua Tint, although I’m guessing it may be a water dye product? If so, it may be best to contact the manufacturer and double-check whether it’s safe to use with fish (and if so, what dosage they recommend for your pond size and stocking levels).

      If you know it’s safe for fish and have applied the correct dosage, it may just be due to the sudden change in water condition after applying the dye. Koi tend to lose some color after any treatment that effects water quality, so I’d maybe give it some time and see if colors returns in a week or so. You could also perform a wide range water quality test in the mean time to make sure your general water parameters are safe, specifically ammonias, nitrates, and pH – all of which can effect color if they’re too high (or even too low, in the case of pH).

  2. My 10 year old koi is losing all its black color. It is now turning all white. Water quality is great; as is the aeriation, and I feed it the best koi food. None of the other koi are affected. he did not experience stress either. I am at my wits end. He is behaving normal and is eating well. Is there anything I can add to the water. I test every week. I have been a koikeeper for 30 years.

    • Hi Della,

      So sorry for the late reply!

      Your fish isn’t stressed, has nutritious food, and healthy water quality. This really only leaves one explanation – genetics. Some fish will simply change color as they age, and it’s perfectly natural and healthy! Just as our hair lightens as we age, so too can a fish’s scales. So long as he’s still acting normal and getting all of the nutrients that he needs, there’s no cause for concern.

      • There is another one which is often overlooked and in my opinion quite common and that is lack of shade and sometimes no shade at all for the fish to hide under. On strong sunny days natural sunbleaching takes affect especially if your pond is shallower than it should be, so folks make sure your pond has adequate shelter.
        Ask whoever does the washing about leaving coloured clothing on the washing line for too long.
        Maintaining strong colours are partly the reason why the opaque waters of Koi mud ponds are so popular in Japan as it reduces the effects of the harmful suns rays.

  3. I have 1 koi and 1 goldfish in a 15 gallon ” pond” I made out of a large planter. They have a filter that is of adequate size which I change regularly. They have a bar style oxygenator. They are by a sliding door I keep,open so they get lots of natural light. They are fed a decent quality food. They seemed happy and active up to a couple months ago. Now they seem uninterested in much food and are losing all of their color. I regularly add fresh water as theirs evaporates and I always de- chlorinate.
    I do not know what to do to improve their happiness.

    • A “pond” of only 15 Gallons is way too small for a koi. They need room to swim and water quality can change rapidly in small tanks. You could buy a 150 or 300 gallon stock tank to keep them in, with external filtration (such as PondMax) and aeration.
      Test your water! Frequent excessive water changes do not allow the water to go through the nitrification process: ammonia to nitrites to nitrates – the last of which may be reduced by floating plants and small water changes.
      I hope your fish survived and you moved them to an appropriate sized pond.

  4. I heard that the color of the pond can influence their color aswell, is this true? If so, can you give a guess of what would happen with a koi in a pond that has light blue walls and bottom?

    • Hi Nery,

      Thanks for reading!

      It’s quite false that the color of a pond will influence the color of koi. At most, a pond with a blue liner may cause a blue coloration to reflect onto the fish, but it won’t actually change their color. The only major things that influence their hue and vibrance are genetics, overall health, and diet.

  5. Hi

    I just built a pond a month ago 10′ wide x 10′ long square. The crescent level around three sides is 2′ deep and the drop off in the center is 4′ deep. the pump is a 2000 GPH with 1.5 hose and the filter is a bio filter with UV able to handle 2000 GPH. I made a water fall that drops into the water 3″above the surface making bubbles at the top and a couple of inches below. I live in NY and I plan to add a heater and bubbler. I have one baby koi (fat) about 4″ and two other koi about 12″ .
    They seem fine, they are ravenous (i feed the every other day) and have eaten all the water hyacinths and whatever frys i had in the pond. I noticed tadpoles or fish fry a week after i let the pond settle right before I introduced them.

    The silver koi had a couple of read spots and a butterfly spot on her crown. That faded to an almost jell like transparency not silver/white like her body and the spots on her body got lighter. The other big koi is predominately white with red and faded black spots. His color hasn’t changed nor has the baby (black with white and orange spots).

    Finally my question… what would make her crown change is it that shes is spawning? (she eats more than the other two pushing them out of the way, even breaching in her eagerness) or is it something else? After rain storms I drop the water 2′, clean the filter sponges but leave the bio balls at the bottom. The reason is after rain the pond gets green. Should I not have crystal clear all the way to the bottom? Some say green at the 4’depth is welcomed.

    Thanks in advance for hearing my long winded questions.



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