Why Are My Pond Fish Dying in Hot Weather? (How To Prevent It)

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four orange and black koi in a sunny pond
If you have pond fish it’s essential to monitor water temperature and water quality, especially during summer months. Public domain.

Water temperature is one of the most important elements impacting the overall health of your pond. The warmer that the water is, the less able it is to hold oxygen; in fact, once water exceeds approximately 85 degrees Fahrenheit, dissolved oxygen levels drop below 5 parts per million (ppm). A dissolved oxygen level of 7 ppm or higher is best for most fish species, and once it drops below 5 ppm, both aquatic plants and animals begin to suffer and die.

Lower oxygen levels means that fish will also have a harder time metabolizing food, fending off illnesses, parasites, and bacteria, and their muscles won’t receive enough oxygen to be able to move about properly and as such fish will become sluggish or may stop moving altogether. Imagine you’re trapped beneath several pounds of heavy blankets, trying to breath but the air inside is too heavy and warm – that is similar to what it’s like for your fish when water temperatures are too high. Warm, hypoxic waters are also prime habitat for algae and bacteria, leading to harmful blooms that further damage your fish.

How Does High Temperature Affect Water Quality?

sunlight and algae on a pond
Sunlight doesn’t just promote algae blooms, it also affects pH, alkalinity and the toxicity of nitrogen compounds. Public domain.

In addition, water temperature impacts pH, alkalinity, and ammonia, all of which are directly tied to the health and survival of your fish. As water temperatures increase, pH generally decreases – as a refresher, a pH level of 7 is neutral, below this is acidic, and above this is alkaline. While the general rule of thumb, according to Le Châtelier’s Principle, is that pH only shifts by a matter of 0.2 per 50° F (or 10° C) water temperature shift, this also depends on the overall “purity” of the water, the nutrients present, and so on. While many species won’t be overly impacted by a shift of 0.2, some might so it’s still a valid factor to keep in mind.

Related to all of this, alkalinity is a solution’s ability to neutralize acids, or in other words the amount of acid that is needed in order to alter the solution’s pH. As temperatures increase, pH drops, meaning, in a very simplified explanation, that water is less alkaline, more acidic, and less able to neutralize those acids. Water that starts off with a relatively high pH of 8.5 or more will be less impacted by temperature shifts…at first.

an ammonia NH3 molecule
The toxic form of ammonia (NH₃) is formed in ponds with high water temperatures and high pH parameters. Public domain.

The quantity of ammonia is primarily dependent on water temperature and pH levels. The toxic form of ammonia, NH₃, increases in tandem with temperature and pH. This means that too high of a pH will result in more toxic ammonia, and too high of a water temperature will also result in more toxic ammonia.

Ammonia toxicity is detrimental to aquatic life, as it builds up in organisms’ systems and causes issues such as convulsions, increased heart rate, eye and gill damage, and even death.

What Are Ideal Water Temperatures for Fish  In Outdoor Ponds?

Suitable water temperature depends largely on the species of pond fish that you have, as well as your location. Here we will cover some of the most popular pond fish species and their water temperature requirements.

Water Temperature for Koi

Koi do best when water temperatures are between approximately 59 and 77 degrees Fahrenheit, or 15 and 25 degrees Celsius.

Water Temperature for Goldfish

Goldfish are most well-suited to a smaller temperature range between 68 and 72° F or 20 and 22° C, but they’re fairly hardy and can tolerate shifts outside of this range so long as it’s not for prolonged periods of time.

Water Temperature for Fancy Goldfish

A range of 68 to 74° F (20 to 23° C) works well for most fancy goldfish. However, there are a variety of subspecies of fancy goldfish, and so you should specifically research the type that you have. For example, lionheads are suited to as low as 65° F (18° C), while fantails do best in temps between 70 and 80° F (21 and 26° C).

Water Temperature for Sturgeon

sturgeon do best in cool water
Public domain.

Sturgeon are very hardy and able to handle temperatures ranging from 50 to 71° F (10 to 22° C), though no more than 59° F (15° C) is best.

Water Temperature for Orfe

The best temperature for golden orfe is 50 to 77° F (10 to 25° C), though they are able to survive in temperatures above this range or below freezing as long as it doesn’t last for several days.

Water Temperature for Plecos

a large common pleco eating algae
Public domain.

Plecostomus are warm water fish, preferring water temperatures between 72 to 86° F (22 to 30° C).

How to Reduce & Control the Temperature of Pond Water (Best Methods)

1) Implementing Pond Plants

plants help shade a pond to prevent algae growth
Keeping around 60% of your surface water shaded by plants and trees can help lower water temperature. Public domain.

Incorporating plants both in and around your pond will help to shade the water, thereby slowing the rate at which it is able to heat up during hot weather. Additional benefits of doing this are that the plants provide valuable food and shelter for fish, aquatic invertebrates, birds, and many others, while also helping to filter the water and add dissolved oxygen. The latter two will also help to prevent algal blooms or nutrient overload from fertilizer run-off, plant and animal decomposition in the pond, and fish food that doesn’t get eaten.

However, you should make sure that no more than 60% of your pond is covered by plants, and no more than the same amount should be shaded by trees. This will ensure that enough sunlight reaches it to allow for proper ecosystem function. A potential drawback is the need to clean your pond regularly to prevent any dead plant matter from depleting oxygen as it decomposes, and also needing to trim back certain species (such as water lilies) so that they don’t grow out of control. As always, ensure that you use only native plant species and work to remove invasive species that could be harmful to your pond and the surrounding ecosystems.

2) Increasing Water Flow  & Aeration 

a small fountain adds aeration to a pond
Extra aeration, through water features and air pumps, will help prevent stagnation and promote heat loss. Public domain.

Keeping your water moving (but not too much) will keep the water consistently filtering around so that there aren’t any overly warm or cold pockets that may shock your fish as they swim about. Possible methods of facilitating water movement include water aerators, waterfalls, fountains, and so on. These will also incorporate oxygen into your pond, and deter algal growth. There aren’t really any drawbacks to this approach, other than ensuring that there isn’t too much water movement, as many pond fish species are adapted to slow-moving water and can become stressed if water is turbid or has too much movement.

3) Adding Shade Sails and Covers

An umbrella over a pond protects pond fish
Shade sails, umbrellas, and covers will directly block sunlight and greatly lower water temperatures. Public domain.

Shade sails are, predictably, modeled after ship sails and are designed to shade variously sized areas, whether they be patios or ponds. Most are able to block up to 90% of U.V. light, and are made of thick, resilient fabric. They can be positioned either directly over the pond for full shade and tensioned into the ground via ropes or tied to surrounding trees, or placed at an angle for partial shade. The temperature below the sail can be as much as 20° F cooler than the surrounding area, thus protecting your fish from potential harm during particularly hot weather. The downside of this method is that your plants will also be shaded out, so this method should only be used for a couple of days at a time to prevent damage to any plants or oxygen deficiency due to reduced photosynthetic activity.

Water covers do essentially the same thing, but are draped directly over the pond itself, and the same logic applies – do not use them for extended periods of time. Both of these methods are better suited to smaller water bodies such as ponds as opposed to lakes.

4) Using Pond Water Dyes

blue pond dye protects fish in a pond
Pond and lake coloured dyes will absorb a percentage of sunlight and decrease the amount that penetrates. Public domain.

Pond dyes are typically used to help control algae, as they block sunlight and therefore hinder algal growth. With this logic, they can also help to control temperature to some degree as sunlight will not be able to penetrate beyond the photic zone, thereby having less impact on its temperature. Blue dye in particular is considered non-toxic to fish (though do make sure that the label says non-toxic as well), and is widely used to help control single-cell, filamentous, and matted algae.

Dyes can be used in both ponds and lakes, but may be less effective/more difficult to use in lakes as the greater amount of water will dilute the dye, meaning that you’ll need to use more in order for it to be effective. The dye will also aid in making your fish more difficult for predators to see, and will clear up in about a month.

5) Maintaining Max Water Levels

keeping ponds full with fresh water lowers temperature
During hot weather, keeping ponds filled to their maximum level will help lower water temperature. Public domain.

During hot weather, you should take care that your pond is as full as possible – deeper water takes longer to heat up than does shallower water. If you need to top your pond off, do so slowly and in increments so as to not shock your fish or make them more susceptible to ich. Do not allow water temperatures to raise or lower by more than 1° F per hour, and only add about 10% to water volume within any 24 hour period, making sure to de-chlorinate water during the process. You can also easily maintain a constant water level in summer months by implementing an automatic-fill system, such as those detailed in our article here!

While this technique works well for ponds, lakes are large and so altering their water level or temperature would take much more time and effort.

Chris G
About the author

Chris G

Pond consultant and long-time hobbyist who enjoys writing in his spare time and sharing knowledge with other passionate pond owners. Experienced with pond installation, fish stocking, water quality testing, algae control and the troubleshooting of day-to-day pond related problems.

Read more about Pond Informer.

2 thoughts on “Why Are My Pond Fish Dying in Hot Weather? (How To Prevent It)”

  1. Just recently my 3 koi fish died. I had them in a 125 gallon above the ground homemade built pond. With a fountain and 2 air stones. A few days before they were acting sluggish and were not eating. I have had them for 6 months. I vacuumed the bottom and added water and added another fountain. Tested water…all in the
    normal range. I bought 2 tiny koi yesterday and added them. This morning, they were dead! What am I doing wrong?

  2. I have an in ground pond about 50 X75 approximately 9 foot deep, Our weather in East Texas has in the 100+ range for 3 weeks and no change in sight. I have mature Cat Fish and Blue Gill in the pond. I went out this morning and many of the cat fish and blue gill are dead and a number of the other cat fish are dying. Changing the water is not an option. The cat fish skin has changed color, instead of being the normal color they color is dull and gray. I think it is the water temperature, my question is the ones that have not died yet but are obviously dying, can they be harvested and eaten before they expire?


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