Hot weather can be brutal on a koi pond. It’s not just about warm water, it’s the knock-on effects like low oxygen, higher waste breakdown, and stressed fish that do the damage. I’ve seen it too many times: lovely koi going downhill fast because the basics weren’t covered when the temperature soared. Whether you’ve been keeping koi for decades or you’re just getting started, these practical tips will help you stay ahead of the heat and keep your pond running steady.
Why hot weather poses a risk to koi
Koi aren't built for extreme heat. They're cold-water fish, and while they can handle a range of temperatures, once things push past 25°C, the risks go up fast. Warm water holds less oxygen, which means your koi have to work harder just to breathe. At the same time, their metabolism speeds up, so they’re using more oxygen and producing more waste. That combo is a recipe for stress, illness, and in worst cases, death.
Heat also makes ammonia more toxic. In cooler water, your filter bacteria can handle the waste load better. But in the heat, bacteria can lag, and your water quality suffers. You might not see the effects straight away, but your koi will feel it, gasping at the surface, clamped fins, or hanging near the bottom. These are all warning signs that things aren’t right.
Bottom line: when the temperature rises, everything gets harder for your koi. That’s why staying on top of conditions is key.
Monitor and control water temperature
You can't fix what you don't track, so start by keeping a pond thermometer in place. Not one stuck on the glass or a guess from your weather app. You want to know exactly what your koi are dealing with. Check it in the morning and again in the late afternoon to get a true picture of the highs and lows.
If your water is regularly creeping above 25°C, it’s time to step in. Shade is your first defence. Trees, pergolas, shade sails, even floating plants like water lilies can all take the edge off. You don’t want to cover the whole pond (koi still need light), but a good 30-50% coverage can bring temperatures down noticeably.
In more extreme cases, you might need to consider a pond chiller or a DIY solution like topping up with cooler water (just make sure it's dechlorinated). I’ve seen breeders in Japan run cool, fresh mountain water through their ponds during summer. It’s simple, but it works.
Don’t rely on guessing or hoping it cools off. Keep an eye on it, act early, and your koi will thank you.
Keep oxygen levels high
Warm water and low oxygen go hand in hand, and it’s one of the biggest dangers during a heatwave. If you see your koi gasping at the surface or crowding around a waterfall or air stone, you’ve already got a problem.
You want plenty of surface agitation. That means air pumps with decent-sized air stones, not just a little bubbler in the corner. If you’ve only got one source of aeration, double it. Run extra lines, boost your air pump, and keep everything clean so it’s working at full capacity.
Waterfalls and trickle towers help too, but only if the flow is strong enough to move the surface. Stagnant water won’t cut it. If you’ve got a shower filter, even better, they’re brilliant for oxygenation, especially in hot weather.
Also, check that your pump isn’t clogged or running slowly. Summer is not the time for equipment failure, so give everything a once-over when the heat kicks in.
Keeping oxygen levels up isn’t just about comfort; it’s survival. Don’t wait until they’re struggling.
Adjust feeding habits in summer
In hot weather, less is more when it comes to feeding. Your koi’s metabolism does speed up with the heat, but that doesn’t mean you should be chucking in food non-stop. Overfeeding is a common mistake, and it’s deadly in summer.
Uneaten food breaks down faster in warm water, pushing ammonia levels up and hammering your water quality. At the same time, your filters are already under pressure from increased waste. It's a double hit that your koi will pay for.
Feed smaller amounts, more often, and watch them eat. If they’re not going for it straight away, stop. Don’t leave food floating around, this does more harm than good.
Also, if your oxygen drops or you’ve had a big storm or temperature swing, hold off feeding entirely for a day or two. Let the system settle. Koi can go a few days without food, and it’s better than risking water problems.
Provide shade and shelter
Shade is one of the simplest ways to take the edge off summer heat. Direct sunlight can send water temps soaring, especially in shallow or smaller ponds. Giving your koi some cover makes a noticeable difference both in temperature and stress levels.
Floating plants like water lilies or hyacinth are great for breaking up sunlight on the surface. They’re not just decorative, they serve a real purpose in keeping water cooler and giving koi a place to retreat. Just make sure they don’t take over the whole surface. You still need good airflow and surface movement.
If plants aren’t an option, go for physical shade. Sail cloths, pergolas, or even a well-placed patio umbrella can help. It doesn’t need to be fancy, just practical and secure enough to handle wind and rain.
In Japan, many breeders use netting or bamboo mats over their ponds during the hottest weeks. It’s all about managing that balance between light and shade. Your koi aren’t trying to sunbathe; they want consistency and comfort.
Avoid overcrowding and manage stress
Overcrowding’s a problem at the best of times, but in hot weather, it’s a disaster waiting to happen. More fish means more waste, less oxygen, and a higher chance of things going wrong fast. If your pond’s overstocked, you’ll struggle to keep conditions right once the heat sets in.
Look at your stocking levels. A good rule of thumb is around 250–500 litres of water per koi, depending on their size. Bigger fish need more space, simple as that.
Stress is another killer. High temps, low oxygen, unstable water…it all adds up. And stressed koi are more likely to get sick. Keep noise and movement around the pond to a minimum, especially during heatwaves. No major changes, no big clean outs - stability is key.
Watch your fish closely. Flicking, sulking, or hanging near the surface can all be signs they’re not coping. Don’t wait until they’re in real trouble.
Stay ahead with regular checks
When the weather’s hot, you can’t afford to take your eye off the ball. Daily checks become even more important. First thing in the morning and again in the evening, take a good look at your koi, test your water, and make sure everything’s ticking over as it should.
Check water temperature, oxygen levels (if you’ve got a meter), and always stay on top of ammonia, nitrite, and pH. Don’t assume your filter will just cope, high temps can throw everything off balance. If your readings are off, act quickly. A small issue in summer can turn into a full-blown disaster overnight.
Clean your filter sponges more often, but don’t overdo it, you want to keep the good bacteria alive. Rinse them in pond water, not tap. Make sure your air stones are bubbling properly, pumps are flowing strong, and nothing’s blocked or struggling.
It’s not about chasing perfection; it’s about spotting problems early. Prevention’s always easier than a rescue mission.
Final thoughts
Summer is meant to be the time when your koi look their best - colours vibrant, energy high, feeding well. But that only happens if you stay ahead of the heat. It’s not rocket science: keep the water cool, oxygen high, feeding light, and stress low. Stick to the basics, don’t cut corners, and watch your fish closely.
I’ve seen too many ponds go sideways in a heatwave because people left it too late or didn’t think it’d get that bad. Trust me, it can, and it will if you’re not ready.
Do the work now, and your koi will come through summer strong, healthy, and ready for the next season.