How can a koi carp get burned? Sun, chemicals, and handling

It is hard to imagine a fish getting sunburned. And yet, one summer morning, some owners discover a pinkish, then grayish patch on the back of their finest white koi. Others see their fish become covered in lesions a few days after building a concrete waterfall. A koi can indeed get «burned», and almost always for one of three reasons: the sun, a chemical in the water, or clumsy handling. All three are fairly easy to prevent once you know how to recognize them.

Sunburn: a real lesion, not a myth

In nature, carp live in more or less tinted water that filters out a large part of ultraviolet radiation. A modern pond, filtered and clarified with UV-C, on the other hand, offers exceptionally clear water. If the pond is shallow and has no shaded area at all, the backs of fish that stay near the surface receive a dose of solar UV radiation they are not adapted to withstand. Sun-related lesions have also been documented in fish farming among fish raised in shallow, highly exposed ponds.

The most exposed koi are those with light-colored skin: the white areas of Kohaku and Showa, platinum Ogon, and very pale Chagoi. A common case: a fish that spent its youth in a muddy pond with brown water and discovers, in June, a crystal-clear, south-facing pond.

The lesion appears on the highest part of the body, along the dorsal area or on the head: first a diffuse redness, then a dull, grayish area, sometimes with peeling skin. It is not contagious, but damaged skin is an open door to bacteria and fungi.

Recognizing the type of burn

Chemical burns: the fresh cement trap

New mortar, concrete or render releases lime when it comes into contact with water. The pH rises, sometimes very high, and ammonium already present turns into ammonia, which is far more toxic in alkaline water, as water-quality manuals in fish farming point out. Result: thick mucus, eroded fins, irritated gills, in all the fish at once. Any new masonry in contact with the pond water must be rinsed, treated or isolated before the fish are returned, and the pH checked for several days in a row.

Other common sources: chlorinated water added in large quantities without conditioner, an overdose of a treatment incorrectly calculated using an overestimated volume, or runoff from weed killer or patio cleaner.

Handling injuries

A koi's mucus is its first line of defense. A dry hand, a towel, a coarse-mesh net, or a fish struggling on a hot surface in the sun can strip away this film and leave an exposed area that is often mistaken for a burn. Catch it with a soft net, transfer it to a basin or floating bag rather than carrying it by hand, and always handle it with wet hands for as little time as possible.

What to do if your koi is burned

1. Remove the cause: immediate shade for sunburn, water changes and pH correction for a chemical burn.
2. Take care of the water: zero ammonium and nitrites, good oxygenation, clean filtration. This is what prevents infection.
3. Observe the wound every day. An area covered with cottony filaments or becoming hollow requires local treatment, preferably with the help of a koi specialist.
4. Feed lightly with a quality food: healing is better in a fish in good condition, without overloading the water.

Prevention: shade, depth and common sense

Aim for one-third to one-half of the surface to be shaded during the hottest hours. In a koi pond, water lilies have a hard time because the fish readily graze on young leaves; planted floating islands, a pergola, a tree planted at a suitable distance, or a shade sail can take over. A deep area, at least 1.20 to 1.50 m, also allows the fish to escape the light. Bonus: shade limits temperature increases and algae growth. Finally, plan masonry work off-season or with the fish sheltered in a tank.

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For the basics of a pond where koi stay healthy: How to care for koi carp?.

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Frequently asked questions

— Can a koi really get sunburned?

Yes, especially in light-skinned fish in a very clear, shallow pond with no shade. The lesion is located on the back and head.

— Does a koi's sunburn heal?

Most often, yes, within a few weeks if you eliminate the exposure and the water remains excellent. A duller mark may persist for a while.

— How long should you wait after concrete work?

There is no universal timeframe: wait until the pH of the water in contact remains stable and normal for several consecutive days, after rinsing or surface treatment.

— Should you turn off the UV-C to protect koi from the sun?

No. UV-C acts inside the unit, not on the fish; you simply need to compensate for the clear water with shade and depth.

In summary

• Three causes: sun, chemical product, handling.
• Sunburn: light-colored koi, clear water, little shade, lesion on the back.
• Chemical burn: fresh cement or overdosing, all fish affected.
• Shade over one-third to one-half of the surface and a deep area.
• Impeccable water to prevent secondary infection.

Last updated: September 29, 2026.