Signs of Heat Stress in Pond Fish

When pond temperatures creep into the mid-90s, goldfish and koi are at risk of hitting their thermal thresholds—the maximum temperature fish tolerate before their body starts to shut down.

“They can only go so hot before they hit their limit,” says Todd Sink, an associate professor and aquaculture extension specialist at Texas A&M University.

Heat stress isn’t always obvious, but there are signs. One of the first is a sudden loss of appetite. “If you go out one morning and suddenly the fish are not feeding, that should be a trigger,” says Sink. You might also notice fish swimming at the surface, swimming erratically, or rolling on their sides.

Our backyard ponds aren’t just decorative—they’re little ecosystems. And when summer temperatures spike, it’s up to us to keep those systems in balance.

Tips to Protect Pond Fish During Hot Weather

Use these simple tips to keep your koi and goldfish safe and swimming, even during the hottest days of summer.

1. Design Ponds with Enough Depth

Some ornamental ponds are too shallow to offer fish a thermal escape. A depth of 3 to 5 feet provides cooler zones that koi and goldfish can retreat to when surface waters heat up. These deeper pockets also help in winter by keeping water from freezing solid.

“A pond must have sufficient depth with cool zones out of direct sunlight for those fish to seek refuge in hot weather,” says Sink.

2. Shade Smartly

Shade helps keep fish cool, but not all shade is the same. While trees block sunlight, they also drop leaves that clog filters and degrade water quality. A better option is to use a shade cloth with 50% to 70% light penetration when temperatures spike, says Sink.

Alternatively, add floating pond plants like water lilies, which naturally block sunlight and help reduce surface warming.

3. Don't Use Pond Dyes

Black, blue, and green pond dyes might look attractive and even help control algae, but they also darken the water, causing it to absorb and retain more heat.

“You risk cooking your fish by adding pond dyes to your ornamental pond,” warns Sink.

4. Run Water Features at Night

Keeping your pond well-aerated is important during hot weather, since warm water holds less oxygen than cold water. But not all aeration is helpful during the hottest times of the day.

Sink advises turning off water fountains and waterfalls during the day when temperatures are at their peak. “They bring the water up to the surface, so it’s exposed to the hot atmosphere,” he explains. That exposure warms the water even more.

Instead, run water features after sunset to boost pond oxygen levels without overheating the pond.

5. Monitor Temperature and Water Quality

A floating pool thermometer—available for around $10—tells you if your pond is getting too hot. If your pond hits the 90°F mark, it’s time to cool things down.

Slowly add cool, non-chlorinated water, like rainwater or well water. Stay away from tap water unless it's been treated with a dechlorinator, and definitely don’t toss in ice cubes made from chlorinated water.

Sink also recommends testing your pond’s water chemistry at least once a week. A spike in ammonia may signal that your beneficial bacteria are struggling or even dying off. Regular testing helps catch these changes early, before they lead to bigger problems for your fish.

6. Adjust Feeding Schedules

Fish don’t need to eat every day. In fact, cutting back during extreme heat reduces waste buildup and oxygen demands.

“One of the very first things we do is withhold feed,” says Sink, describing common aquaculture practices.

Try feeding every other day, or switch to late-evening feedings when water temperatures are lower. Avoid early-morning meals because that's when oxygen levels are at their lowest.

“Feeding early in the morning causes a further oxygen deficit,” says Sink.

7. Consider Native Fish for Your Pond

Koi and goldfish are popular for their colors and puppy-like demeanors, but they’re not native or heat-adapted. They didn’t evolve to deal with extreme heat and cold, says Sink.

He notes that more pond owners are now experimenting with colorful native species like shiners, darters, and sunfish. These fish are not only beautiful, but they’re also naturally built to handle regional temperature swings.

According to Sink, managing exotic species is more challenging.