Carp Pox on Koi: What Is This and What Can I Do?

If you are suddenly seeing white waxy patches on your koi as the water cools down, you are not alone.

"What is this white stuff on my koi?"

Every fall and early spring, our inbox fills up with the same kind of message:

"Hi, can you tell me what this is on my koi? It looks like white wax on the fins and body. Is it fungus? Is my fish dying? What can I do?"

If that sounds like you right now, take a breath. In many cases, what you are seeing is something called carp pox (also known as koi pox). It looks scary, especially when it suddenly appears as the water cools down, but it is usually more of a cosmetic problem than a death sentence.

Q: What exactly is carp pox?

Carp pox is a viral skin condition that affects koi and other carp. Once a fish has been exposed to the virus, it can carry it for life, and the symptoms often come and go with the seasons.

The important point for most hobbyists is this: carp pox is usually not a "fish killer." Many koi live long, normal lives with occasional flare-ups when the water is cold and their immune system is a bit slower.

How carp pox usually looks on your fish

Here is how customers typically describe it, and what we look for in photos:

• Smooth, waxy, white, pink, or translucent patches
• Looks like drops of candle wax, dried glue, or melted plastic on the skin or fins
• Often starts as small spots that can grow together into larger plaques
• Usually flat or slightly raised, not fuzzy or cottony
• Common on fin edges, the dorsal fin, tail, or along the sides of the body

As the water warms back up in late spring and summer, these patches often shrink, smooth out, or almost disappear. The virus is still there, but the koi’s immune system and skin turnover improve with warmer water.

Q: Why does it show up when it gets cold?

You are not imagining it. Carp pox is very often a cold-weather problem. When the temperature drops:

• The koi’s immune system slows down
• Skin and mucus turnover are reduced
• Underlying viral issues that were "quiet" in summer start to show on the surface

On top of that, seasonal stress (fall leaves, unstable water parameters, changing feeding) can make things worse and trigger flare-ups.

Q: Is my koi in immediate danger?

Most of the time, the answer is no. Carp pox is usually cosmetic. What we ask customers to look at first is not just the spots, but the koi’s behavior:

• Is the fish still eating?
• Is it swimming normally with the group?
• Breathing comfortably (not gasping or sitting on the bottom)?

If your koi is acting normal and the white patches match the “waxy” description above, you are probably dealing with carp pox or a similar low-grade viral issue.

We get more concerned when:

• Lesions are on or inside the mouth and the fish struggles to eat
• Lesions interfere with the gills and breathing
• Patches crack open and turn into red, bloody areas or deep ulcers
• Multiple fish are sick, lethargic, or dying in a short period of time

Q: Could it be something other than carp pox?

Yes. There are a few things that can look similar in photos, especially when you are just scrolling through Google images. When customers send us pictures, we are usually ruling out three main “look-alikes”:

1. Lymphocystis (viral, but different)

Lymphocystis is another viral disease that can look a bit like carp pox at first, but up close it has a different texture:

• Pink or white cauliflower-like nodules
• Bumpy, wart-like clusters that look like tiny grapes or popcorn
• Often on fins, but can be on the body or around the mouth

Like carp pox, there is no "magic medication" that kills the virus. The focus is on excellent water quality, low stress, and letting the fish's immune system do its work. Many lymphocystis cases slowly improve over time when conditions are right.

2. True fungus (Saprolegnia)

If the patch looks fuzzy or cottony instead of waxy and smooth, we start thinking about fungus instead of carp pox:

• Soft, cottony, white/gray growth that waves in the water
• Often starts on a wound, damaged fin, or area where scales are missing
• Can spread quickly in cold, dirty, or poorly circulated water

3. Bacterial ulcers or plaques

Bacterial infections can also create light or whitish areas, but they usually have one or more of these:

• Red, inflamed edges or bloody streaks
• Open sores, missing scales, or raw-looking skin
• Fish off feed, hiding, or looking obviously miserable

These are not "watch and wait" problems. They may require proper wound care and, in serious cases, treatment under the guidance of a fish veterinarian.

What about ich and KHV?

We also get questions about:

• Ich (white spot disease): looks like many small, pinhead-size spots sprinkled over the fish, not big waxy patches.
• KHV (Koi Herpesvirus): a different virus from carp pox that causes severe gill damage, lethargy, and high mortality. If fish are dying quickly, treat it as an emergency and contact a fish health professional.

Q: What can I do right now if I think it is carp pox?

Because there is no direct medication that removes the virus from the fish, our approach with customers is to focus on everything we can control:

1. Check and stabilise your water

Viruses flare more when koi are stressed. And nothing stresses koi like bad water. Start by testing:

• Ammonia and nitrite (goal is 0 ppm)
• Nitrate (keep as low as reasonably possible)
• pH (steady, not swinging morning to night)
• KH/alkalinity (we like 120–150 ppm, especially with bead filters)

If you are doing water changes with tap water, or you are fighting ammonia and nitrite, products like these can help:

• Aqua Meds H2O Neutralizer or Dechlor & More to instantly detoxify chlorine and chloramine, and to bind ammonia coming in with tap water.
• De-Tox Plus when you need the “big gun” for binding ammonia and other toxins in heavily stocked or stressed systems.
• Buff-It-Up to raise and stabilise KH so your pH stays steady and your biofilter can do its job.

2. Support the koi’s immune system

Carp pox is a viral issue. Your koi need a strong immune system and a clean environment to keep it pushed back into the “cosmetic nuisance” zone instead of a serious problem. Helpful steps include:

• Keeping stocking density reasonable
• Providing good aeration (air stones, diffusers, or waterfalls)
• Feeding a consistent, high-quality diet appropriate for your water temperature

A mature, healthy biofilter and low levels of harmful bacteria in the water column also make a big difference. That is where Aqua Meds microbial products come in. Products such as MedZyme, MedZyme DC, Pond Support, Summer Blend, Arctic Blend, and Dual Blend are designed to:

• Compete with harmful bacteria and lower background infection pressure
• Support the biofilter so it can handle waste more efficiently
• Reduce sludge and organic waste that fuel disease problems

3. Do not scrape or cut off the waxy patches

One of the first things people ask is, "Should I scrape this off?" Our answer is almost always: please do not.

• Picking or cutting at lesions does not remove the virus
• It can leave raw, bleeding areas that easily turn into ulcers
• It adds a lot of stress at the worst possible time

If a lesion cracks and becomes ulcerated, focus on gentle wound care, excellent water, and, when needed, topical or systemic treatments guided by a fish vet. Aqua Meds wound-care products like Armor and AquaBalm are designed to support healing once an actual wound or ulcer is present.

4. Think about quarantine and long-term biosecurity

Both carp pox and other viral diseases can move from fish to fish. While you cannot completely sterilise a pond, you can lower risk by:

• Quarantining new koi in a separate, fully cycled system whenever possible
• Using separate nets and bowls for quarantine vs. your main pond
• Avoiding sharing water or equipment between ponds without proper disinfection

Q: When should I get a koi vet involved?

There is a point where you should move beyond photos and internet advice and contact a qualified fish veterinarian. We recommend seeking professional help if:

• Koi are having trouble breathing or eating because of lesions
• Lesions are rapidly spreading, turning red, or becoming deep ulcers
• Multiple fish are sick, lethargic, or dying in a short time
• You suspect KHV or another serious systemic disease

A koi vet can do skin/gill scrapes, biopsies, and lab tests to confirm exactly what is going on and build a treatment plan tailored to your pond.

Bringing it all together

If you are seeing white, waxy patches on your koi as the weather cools, you are not alone—and in many cases, you are looking at carp pox, a common, mostly cosmetic viral issue that flares under stress and cold water.

The key steps are:

• Look at how the fish is acting, not just how it looks
• Get water quality right and keep it stable
• Support the immune system with good nutrition and a clean, well-aerated pond
• Resist the urge to scrape or cut off lesions
• Get professional help when behavior or mortality tells you this is more than a cosmetic problem

Aqua Meds products are designed to help you control the parts of the equation you can control: water quality, biofilter health, and background bacteria levels. Combine that with good husbandry and, in most carp pox cases, time and warmer water will do the rest.

This article is for general information only and is not a substitute for diagnosis and treatment by a qualified fish veterinarian. When in doubt, test your water, take clear photos, and reach out for expert help.