Koi Health · Pond Care
How to Tell If Koi Are Sick: 7 Warning Signs to Watch For
Learning how to tell if koi are sick is one of the most valuable things a pond owner can do. Knowing what to look for, and acting fast, can be the difference between a quick fix and a heartbreaking loss.
By Jesse Karbowski · Atlantis Water Gardens · Updated April 2026
Knowing how to tell if koi are sick before things spiral is one of the most important skills any pond owner can have. Your koi have been with you through seasons, storms, and everything in between. They know your face, they swim to greet you at feeding time, and they’re as much a part of your backyard as the pond itself. So when something seems off, that gut feeling you get? Trust it.
The good news is that koi almost always show symptoms before things get serious. The key is knowing what you’re looking for. Here are the seven warning signs every New Jersey pond owner should know, and what to do when you spot them.
TL;DR
If your koi are isolating, not eating, gasping at the surface, flashing against rocks, showing physical changes like sores or bloating, fading in color, or swimming erratically, something is wrong. Your first step is always a water test. Most koi illness starts with poor water quality. When in doubt, call a professional before things get worse.
1They’re isolating from the group
Healthy koi are social. They school together, move together, and feed together. When one fish starts hanging back, sitting at the bottom alone, or hovering near the waterfall away from the rest, that’s the first and most reliable early warning sign that something is off.
Don’t wait to see if it resolves on its own. Isolation is how koi tell you they’re not feeling well before the symptoms become visible. The earlier you act, the better your outcome.
2They’ve stopped eating
Koi are enthusiastic eaters. If your fish are normally rushing to the surface at feeding time and suddenly one or more are ignoring food entirely, take notice. Loss of appetite is one of the most consistent early signs of illness, whether that’s parasites, a bacterial infection, or poor water quality stressing the fish.
Skip a feeding and observe. If the behavior continues the next day, it’s time to test your water and take a closer look at the fish themselves.
3Gasping at the surface or rapid gill movement
If your koi are hanging near the surface with their mouths opening and closing, or their gill movement looks faster than normal, they may be struggling for oxygen. This is urgent. It could mean:
- Low dissolved oxygen in the water
- Ammonia or nitrite spike affecting the gills
- Gill flukes or bacterial gill disease
- Filtration failure
Test your water immediately and check that your aerators and filtration are running properly. This is one of the faster-moving symptoms and should not be left overnight without attention.
4Flashing, rubbing, or scratching against surfaces
Flashing is when a koi darts quickly and rubs its body against rocks, the pond liner, or other surfaces. It looks almost like a quick shimmy or a sudden burst of erratic movement. It’s their way of trying to scratch an itch they can’t reach.
This behavior is almost always a sign of parasites, including flukes, fish lice, or anchor worms. It can also be triggered by a sudden pH swing after heavy rain or a water change. Either way, it warrants a water test and a close inspection of your fish.
5Visible physical changes on the body
Get into the habit of actually looking at your fish, not just watching them swim. Physical signs of illness include:
- Red streaks or sores on the body or fins
- White spots that look like grains of salt (ich)
- Fuzzy or cottony patches (fungal infection)
- Clamped fins held tight against the body
- Cloudy or sunken eyes
- Raised or pinecone-like scales (often a sign of dropsy)
- Visible worms or parasites attached to the skin
Any of these visible signs mean the illness has already progressed past the early stage. Act quickly and consider isolating the affected fish in a quarantine tank to prevent spreading to the rest of the pond.
6Color fading or changes in appearance
Koi are known for their vibrant, striking colors. When those colors start to dull, fade, or darken noticeably, it’s a sign the fish is under stress. Color changes are subtle and easy to dismiss, but they’re worth paying attention to, especially when combined with any of the other symptoms on this list.
Stress in koi is almost always environmental. Start with a full water test covering pH, ammonia, nitrite, nitrate, and dissolved oxygen levels before looking for other causes.
7Erratic swimming or difficulty staying balanced
A koi that’s swimming sideways, spiraling, listing to one side, or struggling to stay at a normal depth in the water may have a swim bladder issue, an inner ear problem, or a bacterial infection affecting their nervous system. This is one of the more serious symptoms on the list and should be addressed without delay.
Isolate the fish if possible and contact a pond professional or fish health specialist. Some swim bladder issues are treatable if caught early.
How to Tell If Koi Are Sick: Start With a Water Test
Once you know how to tell if koi are sick, your next move is always a water test. Poor water quality is the single leading cause of koi illness in New Jersey ponds. Ammonia spikes, nitrite buildup, low pH, and oxygen deficiency will stress your fish until their immune systems can’t fight off what they’d normally handle easily.
A water test takes minutes and tells you immediately whether the environment is the problem. If your water parameters are off, address those first. Many early-stage symptoms will resolve on their own once water quality improves.
If your water tests clean and the symptoms persist, that’s when you look at parasites, bacterial infections, or other disease-specific treatments. And if you’re ever unsure, call us. That’s what we’re here for.
How do I know if my koi is sick or just sleeping in Rockaway, NJ?
Koi don’t sleep the way we do, but they do slow down, especially in cooler water. If your fish is resting near the bottom but moves normally when disturbed and eats at feeding time, it’s likely fine. A fish that won’t move, won’t eat, or has visible physical changes needs attention.
Should I quarantine a sick koi in my Denville pond?
Yes, if possible. Moving a sick fish to a separate quarantine tank prevents illness from spreading to the rest of your pond while you treat the affected fish. It also lets you observe and medicate more precisely without disrupting your pond’s ecosystem.
What causes koi to get sick in New Jersey ponds?
The most common causes are poor water quality, introducing new fish without quarantining them first, seasonal temperature swings, overcrowding, and inadequate filtration. Spring and fall are the most vulnerable times of year for koi in Morris County as temperatures fluctuate and immune systems are taxed.
Can sick koi spread illness to other fish in my Parsippany pond?
Many koi diseases, including parasites, bacterial infections, and ich, are highly contagious in a shared pond environment. Isolating affected fish quickly and treating the full pond when multiple fish show symptoms is critical to preventing a full outbreak.
When should I call a pond professional about sick koi in Bergen County?
Any time you’re unsure, call. Especially if symptoms are progressing quickly, multiple fish are affected, your water tests clean but fish are still declining, or you’re seeing serious physical symptoms like dropsy or open sores. Early professional intervention almost always leads to a better outcome.
Not sure how to tell if koi are sick or what you’re seeing? Don’t wait and wonder. The team at Atlantis Water Gardens has been caring for koi ponds across New Jersey since 2000. We’re happy to talk through what you’re seeing and help you figure out the right next step.
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Written by Jesse Karbowski · Atlantis Water Gardens · Serving New Jersey since 2000