Koi Sunburn Treatment Guide (How To Prevent)

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When exposed to too much direct sunlight/UV light, just about any animal has the potential to get sunburned. Those that lack protective fur, such as fish, are most at risk. In fact, fish skin is more sensitive to sun damage than is human skin! As discussed in a study conducted on both wild and farmed fish, “UV carries more energy per photon than any other wavelength reaching the Earth’s surface,” meaning that it has great potential to damage skin not only at the surface level, but DNA and proteins as well. This can cause lasting skin and scale damage (such as painful lesions and complete necrosis of patches of skin), and in extreme cases sickness and even death.

Many fish species, including koi, possess a slime coating on their skin that acts as a photo-protective barrier from UV rays, but this coating does not seem to be evenly distributed across the fish but rather concentrates itself on more sensitive areas (such as around the gills), and its thickness is influenced heavily by water quality and diet. As such, regardless of this protective coating, there is potential for your koi to become sunburned.

In addition, worldwide there has been an increase in fish and amphibian mortality from UV exposure due to deteriorating ozone layer conditions. White or light colored koi are more likely to be impacted by sun damage, as their skin lacks protective melanin. However, there are steps that you can take to recognize the signs of sunburn in your koi and protect them.

What Are Symptoms of Sunburn in Koi?

Just like with us, there are varying degrees of sunburn in koi and within those, different symptoms to watch out for.

Minor Sunburn

a white and orange koi with sunburn
Koi with minor sunburn. Photo by TaylorNews, CC BY-SA 3.0, via Wikimedia Commons..

The most common symptom associated with minor sunburn is a slight pink hue that may look sort of like a rash, particularly on your fish’s dorsal (back) area. It could look like your fish was hit or scratched by something. There could also be some slight swelling, and your fish may be warm rather than cool to the touch. Your fish may also display a slight loss of appetite and experience behavioural changes (for example, if you have a more animated fish, it may become more sluggish with the discomfort, and if your fish is more docile, it may become more erratic with sunburn – it truly depends on the fish). As this progresses into more moderate sunburn, you’ll notice your fish trying to swim to deeper water depths with cooler water.

Moderate Sunburn

If your fish continues to be exposed to an excess of UV light, the pinkish hue will progress into a deeper red color. Their protective slime coating will also begin to dry up and flake off, as it occurs largely in their outer skin layer that is most easily harmed by UV light, which in turn makes them more prone to sun damage. If the issue is remedied in time, fish will be able to regenerate their slime coating, but if left too long, your fish’s ability to produce this photoprotective layer may be lost entirely as damage to their body worsens. Lesions may begin to form in this moderate stage.

Major Sunburn

Major sunburn constitutes a significant risk to your koi’s life. By this stage, their slime coating is entirely gone (and perhaps unable to replenish itself at all), lesions have progressed to blisters and ulcerations, and these in turn open up the fish to all manner of secondary infections and disease as their bodies are weakened. In this stage, the risk of death is substantial.

How to Prevent Sunburn in Koi, Goldfish & Pond Fish (Best Methods)

1) Provide Ample Shade

plants provide shade and protect fish from sunburn
Providing extra shade, such as with floating plants, can provide places for fish to hide. Public domain.

If you notice your koi suffering from sunburn or your pond is in a very sunny area, providing shade is one of the easiest ways to mitigate or eliminate sunburn risk altogether. This can be done by adding fish shelters to your pond, as well as real or fake hollow logs, PVC piping, hollow rocks, etcetera that they can hide in to protect themselves from the sun. You can also incorporate more plants, both in and around your pond. Tall trees will provide shade through at least part of the day, while large-leaved aquatic plants like lilies will provide direct shade throughout that day. Deeper aquatic plants, like hornwort, work to provide shade below the surface in the pond as well.

While incorporating plants is the preferred method here as they provide a host of other benefits as well (such as filtering your water and providing dissolved oxygen), you can also purchase a shade sail and install it over your pond as well. Regardless, you should make sure that no more than 50 to about 70% maximum of your pond is shaded, as totally shading it will limit photosynthesis and natural oxygenation.

2) Increase Pond Depth

koi in shallow water are more susceptible to sunburn
Although not always easy, increasing depth in shallow ponds by just a few feet can hugely increase UV protection. Public domain.

Predictably, sunlight has a harder time penetrating the deeper a body of water is. While light will be able to reach even the depths of your pond to a degree (unless it’s several hundred meters deep, which is quite unlikely!), it becomes significantly more diffuse beyond the first 3-4 feet of water. Therefore, increasing the depth of your pond to greater than four feet in depth will help protect your fish from sunburn, as they’ll be able to seek these safe, cooler, dimmer deeper waters to escape the sun when it’s burning too bright and hot for their liking and safety.

3) Use Pond Dyes

Blue pond dye reduces UV penetration and sunburn in fish
Adding water dyes can help reduce UV penetration in shallow ponds by absorbing energy from light. Public domain.

Fish-safe pond dyes are a great way to help block out the most damaging UV rays, particularly if adding more plants or a sail for shade aren’t viable options for you. The upside is that these dyes can also help deter algal growth as they limit the amount of sunlight that can penetrate the water’s surface; the flip side of this is that it also limits sunlight for any plants that you do have, and can hinder their growth as well as the amount of oxygen that they release. You will also have to reapply the pond dye every so often, as it of course won’t last forever.

4) Perform Regular Water Changes

water changes treat sunburn by improving water quality and introducing cooler water regularly
Water changes help improve water quality, but also introduce cooler, fresher water during hotter months. Public domain.

During particularly powerfully sunny spells, changing out your water may be a beneficial step to take. If your pond water is above about 75° F (23° C), performing a partial water change will not only help prevent sunburn, but will improve water quality overall since warm water is able to hold less dissolved oxygen than cooler water. Water changes need to occur slowly, over the course of several hours or even the entire day to limit the potential to shock or otherwise harm your fish. In addition, don’t change out all of the water, as doing so will be too much of a shock for your fish. Nonetheless, draining out some of your old, warmer water with fresher, cooler water will in turn help keep your fish cool and reduce the likelihood of them developing sunburn.

5) Maximize Aeration

Aeration oxygen keep water temperature cooler and reduce sunburn in fish
Providing large amounts of aeration will help reduce water temperature and better circulate the system. Public domain.

More aeration means more oxygen in the water, and more oxygen in the water means that your fish will be able to move about freely in your pond. In low oxygen waters, fish stay closer to the surface where there is more oxygen, and this of course makes them more susceptible to sunburn. Therefore, having well-oxygenated water will enable them to move to deeper waters for protection while still being able to breathe properly. Aeration can easily be provided using air compressors (recommended for fish), but can also be achieved with a combination of fountains, waterfalls, pumps, and good pond design.

How to Treat Sunburn in Koi & Goldfish

high quality fish food help fish heal from sunburn
A high quality fish feed will help fish recover, and should be maintained for the best health in future.

Depending on how severe your koi’s sunburn is, it can take a few weeks to properly heal, particularly if their slime layer and scales were damaged significantly. If sunburn is only minor and in the beginning stages, simply providing them with shade, cooler water, and high-quality food (as nutrients will aid in the healing process) combined with some time should be enough for them to recover.

With more moderate or severe sunburn, antibiotics and medicated dips and baths (conducted several times throughout each day) or fish-safe pond healing treatments may be necessary to treat lesions that could otherwise become infected and kill them. Quarantining may also be needed, to keep them as calm and comfortable as possible while keeping them in an area that is cool with as little light as possible until they’ve healed from the worst of it.

In extreme cases, or if the above isn’t working, consulting an experienced vet may be in your (and your koi’s!) best interest, particularly if you don’t notice any change after two or three days. They may prescribe stronger antibiotics, or administer direct injections that would be difficult for most pond hobbyists to administer to their fish themselves.

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