Texas Cooperative Extension,
Texas A&M University, College Station, Texas

July-August 2004

Avoiding a Fish Fry: Hazards of warm Water in Your Pond

Reprinted from Aquascape Designs, Inc

Larger Koi
Larger koi have greater demands
on the pond environment


Water gardens are often and correctly perceived as cool oases. In fact, cool water in a pond is more than just aesthetically pleasing. A pond's water must be cool for the fish and plants to thrive. High water temperatures are unhealthy for a water garden, and can even be deadly. The good news: hot water problems seldom happen overnight. Emergency steps can reverse water heating troubles. And for the long term, some simple techniques can help ensure that your pond water stays cool and under control.

Water doesn't need to reach the boiling point or even feel hot for it to cause stress or even real harm to your pond life. In fact, if the water temperature gets above 80F it's already too hot for most fish and no fun for your plants, either.

One sure sign of trouble is fish gasping for air close to the water's surface. Warm water has a low capacity for holding oxygen, while cooler water is usually oxygen-rich. For fish, warm water and increased activity go hand in hand. But with all that activity, the fish require more oxygen, just when less is available.

Biological activity increases with warm water. In high-temperature/low-oxygen conditions fish are more susceptible to infection and disease, because these are the conditions in which bacteria and parasites thrive.

Fish aren't the only living things that are negatively affected by warm water conditions. Plants also can suffer from high heat. Even the leaves of hardy waterlilies (Nymphaea) may begin to burn and turn brownish in extreme heat since they are cold season plants.

But - it is imortant to remember that heat damage rarely happens quickly. It normally takes a long time for a pond to warm up to a deadly temperature. Befor it does, there are things you can do to beat the heat.

Beating the Heat in an Emergency

If warm-water is already a problem in your pond, don't delay. You can save your plants and fish if you give them the help they need before it's too late.

Here are some quick emergency measures:

  • If there's an easy way to create instant shade over your pond, do it.

  • Do a partial water change and add cooler water. If you change more than 20% of the water, you will need to use a dechlorinator, a product that removes the chlorine added by municipal water supplies. Most cities treat water with chlorine, some cities are transitioning over to chloramines (chlorine and ammonia) which is a longer lasting disinfectant. Make sure to use a complete water treatment product which removes both chlorine and chloramines, as well as any ammonia left behind by the breakdown of chloramines. If you use well water, of course, you don't need any treatment.

  • Boost the circulation of water. Even the smallest of ponds can handle a 3,000 gallon/hour pump. If your pump is rated less than 3,000 gallon/hour, install one that is.

  • Larger ponds too can suffer low water flow. Make sure your pump is up to the job. That will keep the water cool and flowing.

  • If yet an additional boost is needed, add an air pump to create turbulence and get more oxygen into your pond's water.

Avoiding High Heat Permanently

If your pond has overheated even once in a typical summer season in your area, chances are it will happen again. You should consider making permanent improvements that will not only lower the heat on your pond, but help make it more beautiful and easier to maintain as well.

Circulation is key to keeping the pond cool. A larger capacity pump can be part of the solution. Adding a waterfall or stream can do more. These elements increase the pond's overall circulation, keeping temperatures down.

Shade is another important long-term fix. Shading your pond can be quick, easy, and beautiful. If your waterfall is exposed to direct sun, add shrubs, trees, a pergola covered with vines or other landscaping features to provide cover and make your water garden look more natural.

Trees grow faster than you think and even a young tree situated in the right location can begin providing your pond with much needed shade immediately. Also a decorative bridge, screening or additional boulders can help to quickly turn a hotspot into a cool refuge.

Of course, having enough aquatic plants is a must. Adding some new waterlilies can be fast, easy and very effective; soon they will be shading a large area of your pond attractively. A good rule of thumb: have at least 40% of your water garden's surface covered in vegetation.

It's also important to keep the pond clean. A thorough cleaning is recommended once a year in four-season climates, and once every few years in warm climates. And make sure you've got plenty of beneficial pebbles and stones on the bottom; these create a friendly environment for good bacteria and help maintain a pond's ecological vitality.

Finally, does your pond have a good-sized space that's approximaaately two feet deep? If not, it may be time to add a section to your pond to provide this necessary "deep" water advantage. Deeper water is always cooler than the water above. As the water circulates through the pond this helps keep the entire ecosystem cool and balanced.

In the end, how cool you can or need to keep your pond will be determined by where you live. A four-season pond actually needs to be cooler than a warm weather, year-around pond. Each will be host to different kinds of plants and wildlife. For example, if you live in a very warm climate, your pond is probably not a good bet for cold-water fish such as koi but might be perfect for tropical fish like swordtail or tilapia [Editor's note: koi respond very well to water temperature conditions in Central Texas]. Warm water fish won't solve everything. Even tropical fish won't thrive when water temperature nears 90F. So no matter where you water garden, knowing how to regulate water temperature is smart pondkeeping.