Heatwave Survival Guide for Koi Pond Owners

 

As summer temperatures soar, koi pond owners face a unique set of challenges. While koi are hardy fish, extreme heat can push even the most well-maintained ponds into dangerous territory. Read on to discover how to best protect your pond during extreme heat!

Oxygen levels plummet, algae blooms flourish, and water evaporates rapidly—all of which can stress or even harm your koi. With some proactive care and a good understanding of how heat affects pond ecosystems, you can keep your koi thriving through the hottest days of the year.

The Critical Role of Oxygen

Warm water holds significantly less oxygen than cool water. This means that during a heatwave, your koi are already dealing with reduced oxygen levels—and as their metabolism increases with the temperature, they need even more oxygen than usual.

To counteract oxygen depletion:

• Add extra aeration: Boost your pond’s oxygen using air pumps connected to air stones or air disks and add fountains for additional surface movement. This is the perfect time of year to add a solar fountain or solar air float! This will increase your oxygen without the need for extra electricity.

• Keep water moving: Stagnant water loses oxygen quickly. Make sure your pump is circulating water efficiently. Make sure the surface of the pond is moving well and there is plenty of flow to allow the gas exchange to occur.

• Avoid overfeeding: Excess food breaks down and consumes oxygen as it decays, further depleting the supply. It can also lead to spikes in ammonia. If unsure how much to feed, remember – less is more!

Early signs of oxygen stress include koi gasping at the surface or gathering near waterfalls or aerators. Don’t wait to act—oxygen depletion can quickly become fatal.

Provide Plenty of Shade

Direct sunlight not only warms the water but also encourages algae growth. Providing shade helps to regulate the pond's temperature and protects your koi from sunburn (yes, koi can get sunburned!).

Ways to create shade:

• Add floating aquatic plants like water lilies or water lettuce. Add floating plant baskets; these can be anchored to the side of the pond for a fixed shade spot or allowed to float freely around the pond.

• Use shade sails, pergolas, or strategically placed plants. These will keep the water cooler and provide cover for your fish.

• Consider adding covers designed for pond use. These can be a temporary fixture or installed permanently.

Stay on Top of Water Loss

Hot weather leads to faster evaporation, which can noticeably lower your pond's water level. As water evaporates, the concentration of toxins from waste increases, putting extra stress on your koi. This becomes especially dangerous if ammonia or nitrite levels rise, as these compounds can reduce oxygen availability in the water more quickly, making it even harder for your fish to breathe.

To manage this:

• Top up regularly with dechlorinated water (use a dechlorinator treatment or inline system when refilling from the hose).

• Use a pond auto-top-up system if you’re away or can’t check daily.

• Monitor for leaks—sometimes heat can worsen small structural issues in liners or plumbing. Regular checks can spot problems early.

Try to top up early in the morning or in the evening to avoid temperature shock from cold water.

Test Water Frequently

Hot weather can lead to sudden shifts in water chemistry, increasing the risk of stress and illness for your koi. During a heatwave, it's crucial to monitor the following closely:

· Ammonia and Nitrite: These harmful compounds can rise quickly, especially if your filtration system is under strain or oxygen levels drop.

· pH & KH Levels: Elevated temperatures and algae growth can cause daily fluctuations in pH. Regular topping up from evaporation can also cause sudden changes to the water parameters which can be stressful for the fish. By keeping your KH level stable it will in turn keep the pH Level at an optimal level. If you need to increase you KH simply add KH Buffer to the pond , this should start to work immediately. 

· Oxygen: Watch for signs of low oxygen, such as koi gasping at the surface. If your pond includes bottom-dwelling species like sturgeon or sterlets, ensure oxygen is circulating well at all levels of the pond—not just the surface.

Test your water at least twice a week during periods of extreme heat—or daily if your fish show any unusual behaviour. Staying on top of water quality is one of the most effective ways to keep your koi healthy in hot weather.

For more information on how to treat common water quality issues click here to read our help guide.

Algae Blooms: The Hidden Danger

Warm water and sunlight are a perfect recipe for algae blooms. Ordinarily, algae in a pond is nothing to worry about, however, during hotter weather this may not be the case. While some algae are harmless in small quantities, a bloom can:

• Steal oxygen at night (algae produce oxygen during the day and consume it at night).

• Block sunlight and alter the pond’s ecosystem.

• Cause pH swings and increase ammonia levels.

Combat algae by:

• Adding more shade.

• Using a combination of beneficial bacteria and live plants to outcompete algae.

• Installing a UV clarifier to keep green water in check.

• Avoiding chemical treatments, which can be harsh on koi and further stress the pond during heatwaves.

For a deeper dive into managing algae issues, check out our blog post: Effective Prevention and Solutions for Algae Problems in Ponds. You'll find practical tips on identifying different types of algae, controlling blooms, and maintaining a healthy balance in your pond year-round.

Be Proactive, Not Reactive

Heatwaves don’t have to spell disaster for your koi pond. With a proactive approach—adding shade, boosting oxygen, managing evaporation, and monitoring water quality—you can help your koi not just survive, but thrive, even during the hottest days.

The key is vigilance. During periods of extreme heat, small changes in your pond can quickly escalate. By staying alert and taking action early, you can keep your koi healthy, vibrant, and stress-free all summer long.

Take Action to Protect Your Pond This Summer

Heatwaves can take a serious toll on your koi pond—but with the right care, you can stay ahead of the risks. Your koi depend on you to keep their environment safe, balanced, and full of oxygen. Whether you’re a seasoned pond owner or just getting started, a few simple steps can make all the difference during extreme heat.

Here’s what you can do today:

• Check your oxygen levels and boost aeration where needed

• Add shade to reduce water temperature and prevent sunburn

• Top up evaporated water—safely and gradually- using a Tap Safe solution.

• Test your water quality regularly to catch and treat issues early

• Prevent algae blooms before they get out of hand

Want to keep your koi happy and healthy all summer long?

Explore our summer pond care essentials in store and on our website for everything you may need to beat the heat! 

Contact us via phone or email for tips and advice on your pond, we’re here to help! Don’t let the heat catch you off guard—your koi will thank you!

For more information about koi or advice on your pond, check out the rest of our blogs…

Understanding Your Water Parameters for a Healthy Pond Ecosystem | Pond Plants: How to Choose the Right Ones and Their Benefits for Your Pond | The Importance of Pond Aeration