Koi Care: The Five Most Common Causes of Sick Koi

Every week, koi keepers reach out asking the same question: “Why are my koi sick?” The good news is that most koi health problems are preventable. In fact, the majority of sick koi cases come back to just a few common mistakes in pond management.

Below are the five most common causes of sick koi, plus simple steps you can take to keep your fish healthy, active, and thriving.

1. Poor Pond Water Quality

By far the number one cause of sick koi is poor water quality. Many experienced pond keepers estimate that well over half of koi health issues start here. Even if the water looks clear, it can still be chemically unsafe for your fish.

Common water quality problems include:

Filtration that’s too small for the pond size or fish load.
Not enough aeration – relying only on a waterfall or small fountain when the pond really needs extra air stones or a diffuser.
Skipping regular water changes – letting dissolved waste, DOCs, and nitrates slowly build up.
Not testing weekly for ammonia, nitrite, pH, and KH (carbonate hardness).
Over-cleaning filters and washing away the beneficial “good bacteria” that process fish waste.
Rinsing filter media with chlorinated tap water, which kills nitrifying bacteria and can cause mini “crashes” in your biofilter.

To prevent water-quality-related illness:

Match your filter size and media volume to the actual pond volume and fish load.
Provide reliable aeration with one or more air pumps and bottom diffusers, especially in warm weather.
Do regular partial water changes (often 10–20% weekly) to dilute waste and refresh minerals.
Use a water conditioner that detoxifies chlorine, chloramine, and ammonia whenever you add tap water.
Test your pond water at least weekly and before and after major changes or treatments.
Rinse filter media in a bucket of pond water, not under the tap, to protect your biofilter bacteria.

Helpful tip: Full-spectrum water conditioners and beneficial bacteria products—such as Aqua Meds® H2O Neutralizer™, DeChlor & More™, and Dual Blend™ or Arctic Blend™—can make it much easier to keep water quality stable, especially during spring startup or after large water changes.

2. Not Quarantining New Fish

Bringing home a new koi is exciting—but it can also be risky. New fish may carry parasites, bacterial infections, or viruses that don’t show immediately. If you add them directly to your main pond, they can expose your entire collection.

Quarantine is absolutely essential.

Best practices for quarantining new koi:

Set up a separate quarantine tank or pond with proper filtration and aeration.
Quarantine new fish for at least 3–4 weeks, longer if you see any signs of stress or disease.
Test water frequently and keep conditions as stable as your main pond.
Observe closely for flashing, clamped fins, sores, excess mucus, or breathing issues.
Have appropriate pond medications and parasite treatments on hand, such as Aqua Meds® Terminate™, Aqua Prazi™, and FlukeGuard™, and treat based on an accurate diagnosis—not guesswork.

A good quarantine routine is one of the cheapest “insurance policies” you can have for your koi collection.

3. Overfeeding

Feeding your koi is fun, and koi are very good at “begging.” Unfortunately, overfeeding is a major cause of poor water quality and sick koi.

Problems caused by overfeeding include:

Uneaten food rotting on the bottom and spiking ammonia and nitrite.
Greasy film or cloudy water from excess protein and fats.
Digestive stress and fatty liver disease in the fish themselves.

Simple feeding guidelines:

Only feed what the koi will eat in 3–5 minutes.
Adjust feeding based on water temperature – much less in cool water and none at very low temperatures.
Use a high-quality koi food appropriate for the season (growth, color, or wheat germ for cool water).
Skip a feeding if the water is cloudy, parameters are off, or the fish seem “off” or sluggish.

Healthy koi with clean, well-filtered water will grow beautifully on moderate, consistent feeding. More food is almost never the answer.

4. Overcrowding (Too Many Fish)

Another very common cause of sick koi is simply having too many fish for the pond size. Overcrowding leads to chronic stress, poor water quality, and a much higher risk of parasites and bacterial infections.

Signs your pond may be overstocked:

Constantly high ammonia, nitrite, or nitrate—even with good filtration.
Fish “piled” together with little room to swim freely.
Frequent outbreaks of ulcers, fin rot, or parasites.

As a very rough guideline, many hobbyists aim for light stocking, especially in smaller backyard ponds. It’s better to have a few high-quality koi in plenty of water than a large number of fish in marginal conditions.

If you suspect overcrowding:

Rehome or sell some fish to reduce the bio-load.
Upgrade filtration and aeration if possible.
Be extra diligent with testing, water changes, and general pond maintenance.

5. “Chemical Soup” – Overusing Medications

When koi get sick, it’s natural to want to help quickly. But adding “a little bit of everything” to the pond can turn your water into a chemical soup that stresses fish even more.

Common mistakes include:

Stacking multiple medications at the same time without knowing if they are compatible.
Switching products too quickly and not allowing time to see whether a treatment is working.
Skipping water testing and water changes between treatments.
Treating the whole pond when only one or two fish are affected and could be moved to a hospital tank.

Safer approach to treatment:

Start with testing. Check ammonia, nitrite, pH, KH, and temperature before you do anything else.
Identify the problem as accurately as you can—ideally with a microscope scraping for parasites or guidance from an experienced koi keeper or aquatic vet.
Use one medication or protocol at a time and follow the directions carefully. For example, if you are treating parasites with a product like Aqua Meds® Terminate™, complete the full course as directed instead of stacking multiple treatments.
Between treatments, support the fish by improving water quality, increasing aeration, and using a trusted water conditioner during water changes.

When in doubt, focus on clean, well-oxygenated water first. Many koi bounce back remarkably once their environment is corrected.

Putting It All Together

Most koi problems are not “mystery diseases.” They usually trace back to one or more of these five issues:

Poor pond water quality
Skipping quarantine for new fish
Overfeeding
Overcrowding
Creating a “chemical soup” with too many treatments

Get these basics right, and you’ll prevent the vast majority of health problems before they ever show up. Your koi will reward you with better color, stronger growth, and years of enjoyment.

If you need help improving water quality, boosting your biofilter, or recovering sick koi, you can explore Aqua Meds® water conditioners (like H2O Neutralizer™ and DeChlor & More™), beneficial bacteria (such as Dual Blend™, Arctic Blend™, and Pond Support™), and parasite treatments (including Aqua Prazi™, Terminate™, and FlukeGuard™) in our online store:

Browse Aqua Meds® products for koi ponds

Always consult a qualified aquatic veterinarian or experienced koi health professional if you are unsure about a diagnosis or treatment plan.