Pond Water Temperature Guide (Best Pond Thermometers)

Water temperature plays a vital role in the overall health of the eco-system and the health of pond fish. Monitoring temperature helps us keep it within safe parameters and is an important aspect of pond keeping. In this guide we explain the ideal temperatures to aim for and explain the best tools for reading water temperature.

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Pond Temperature Change – What Causes It?

As the seasons change, predictably so does water temperature. In the height of winter you can expect your pond to be cooler than in summer; the colder the air, the more heat will be lost from the surface water. However, in addition to this, deeper water bodies also experience something called thermal stratification and turnover. Shallow water bodies don’t undergo a great deal of turnover; it’s more common in lakes or large ponds at least a few meters deep, such as big koi ponds. If you have a very shallow or small pond, the water temperature will tend to decease (or increase) together with the average air temperature throughout the year, which is why smaller ponds can literally freeze solid in colder weather!

For larger ponds or lakes, in summer months the cooler water will stay on the bottom (called the hypolimnion) of the pond with warmer water on top (called the epilimnion). The hypolimnion will contain more dissolved oxygen, and so during warmer months you can expect to find your fish spending more time toward the bottom of your pond where oxygen is more readily available. During autumn, the water will “turn over” due to cooling – cold water is more dense, and will sink, forcing the now warmer hypolimnion water to rise. This process takes some time and results in a relatively even temperature until the water gradually settles into winter stratification where warmer water is located at the hypolimnion and cold water is frozen at the top of the epilimnion. If the water is deep enough, it will not freeze entirely and some fish species will be able to survive by hanging out in the deeper portions of the water. See digram below for further information:-

During spring, the water will again turn over, followed by summer stratification. Because of this, you must monitor your water temperature year-round at all depths (not just at the surface) to ensure you get the most accurate temp readings!

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Overall, warmer water cannot hold as much oxygen. In fact, water that is above 85° Fahrenheit (30°C) is not able to hold much oxygen at all, as the solubility of oxygen gas decreases as temperature increases. At 32°F (0°C), fresh water is able to hold approximately 15 parts per million of dissolved oxygen; at 77°F (25°C), this drops down to 8 ppm. Research has found that, overall, fish need at least 7 ppm of dissolved oxygen to fully support comfortable activity with minimal stress. As such, warmer water (over 80°F, generally) contains less dissolved oxygen, which in turns results in sluggish fish with slower metabolisms as they enter a torpor-like state to conserve oxygen. Similarly, water that is too cold (below 50-60°F, depending on the species) will result in the slowing of digestive enzymes and possibly trigger early hibernation.

For example, goldfish operate best when water temperatures are between approximately 68-72°F (20-22°C). Though they are considered fairly hardy and can tolerate moderate temperature fluctuations, their metabolism will slow concurrently with decreasing water temperatures. They are able to enter torpor and survive overwinter if water does not entirely freeze, but they should still be moved indoors if there is a risk of the water freezing at all. Koi have a greater temperature comfort range of 59-77° Fahrenheit (15-25°C), though they also do not tolerate rapid fluctuations, and can even enter an early state of hibernation during particularity cold spells.

Regardless of the fish species, water that varies too quickly by even a few degrees Fahrenheit can cause shock, paralysis, illness, and even death. This is why it is recommended, for example, that pet fish be placed in aquariums or ponds in a water-filled baggie first so that the water in the bag gradually becomes the same temperature as the aquarium. This allows the fish to properly acclimate to the temperature before being released directly into the aquarium. This same principle applies to fish and other aquatic animals in nature and in your pond.

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As mentioned previously, warmer water contains less dissolved oxygen. This not only affects your fish, but any other organisms present, from plants to bacteria to reptiles and amphibians. The less dissolved oxygen there is, the more these organisms will struggle to survive. In extreme cases, high water temperatures with little water flow and low oxygen will result in fish, plants, and so on dying off, which will further deplete oxygen levels as decomposition occurs, and could lead to algal blooms. This is known as a “summer fish kill.”

As a general rule of thumb, waters that average near or above 80°F (27°C) hold less biodiversity than a water body that is, say, 70°F (21°C) due to there being too little dissolved oxygen in higher-temperate water. Few organisms, fish or otherwise, are able to survive and reproduce in waters that have less than 6 parts per million of dissolved oxygen. When water temperatures exceed 85°F (30°C) and drop below 5 ppm of dissolved oxygen, plants and animals alike begin to die.

In addition, as water temperature increases pH tends to decrease. Low pH is considered acidic, 7 is neutral, and above that is alkaline. Conversely, as water temperature decreases, pH increases. Alkaline water increases the toxicity of water, and kills off or reduces the efficiency of nitrifying bacteria. Without these bacteria, ammonia cannot be broken down and water can become toxic to fish. These immensely helpful bacteria operate best at a pH range of around 7-8; to an extent, they can operate above or below this range but will break down ammonia much more slowly.

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For common pond species, during the summer, you should aim to keep your water between approximately 60 and 75°F (16-24°C), as this will accommodate a fair number of species such as goldfish, koi, orfe, and various algae eaters. In winter, try not to let the water drop below 50°F (10°C). If this happens and your pond is less than a few meters deep, you should move your fish into an indoor aquarium to safeguard their health and survival. Pond fish can survive the frozen surface in a larger pond by retreating to the deeper layers, but cannot survive a pond that freezes solid from top-to-bottom. If your pond is prone to freezing solid, you will need to bring fish indoors or install a water heater!

To keep track of the temperature, you can simply utilize a water thermometer. Remember, deeper water will be a different temperature than surface water, and this also differs depending on the season. Therefore, it is best to utilize a floating water thermometer to measure the surface water temperature, and a submersible thermometer to monitor deeper areas. To foster greater accuracy and calculate an average, check the water temperature both at midday (when temperatures are highest) and at night or in the morning when temperatures are cooler. It would also be in your best interest to keep an eye on pH levels using a water quality test kit, particularly during the warm summer months.

Floating thermometers are probably the easiest way to directly measure water temperature, and come simply as floating units or wireless units with an external display sensor. This type of thermometer is designed to float on the surface of the pond and take a temperature reading just below the surface water

. If you don’t purchase one with an external display for reading temperature, you will need to manually remove the thermometer every-time you want to take a reading, which sadly can affect the measurement. For this reason, we recommend a floating thermometer with an external temperature display which you can keep close-by to get easier (and more accurate) temp readings. Not only are floating thermometers with a display easier to read, they often come coupled with built-in alarm systems so you’ll know exactly when temperatures get too high or low in your pond!

The only downside of these type of thermometers is that they can only measure temperature just below the surface, so are best used for ponds 0.5-2 meters deep or alongside a digital probe thermometer (below).

2) Digital Probe Thermometers 

A second type of thermometer is the digital probe, which allows for deeper temperature readings in comparison to a floating thermometer. Unless you’re looking to measure the temperature of a lake, most koi ponds (which average 1-3 meters of depth), will only require a probe length of 1m to get an accurate “deep layer” temperature reading. Due to this, most digital probe thermometers designed for larger aquarium application should be more than sufficient for pond use, and will help save you money as they’re generally cheaper than specialised devices.

When looking for a device, you’ll want to ensure the cable/probe length is a minimum of 1 meter and that the device has readings in your system of measurement (°C/°F). When measuring, you’ll want to dip the probe into multiple pond positions to get an average overall reading, holding the probe in place until the temperature stabilizes on the display.

3) Infrared Thermometers

A final type of thermometer suitable for reading water temperature are infrared devices. Although they look like something from a sci-fi movie, they’re actually fairly simple devices, using a beam of infrared light to directly measure surface temperature of the object in their path. The advantage of these devices is that they can provide a very accurate and fast temperature reading , which is why they’re popular within the engineering and electrical industry. However, the infrared beam is not designed to read temperature below the surface of an object, so you can only expect an accurate temperature reading of your surface water and not much else.

An upside is that they can also be used outside of water temperature measurement, and can be useful for reading the temperature of various pond equipment, such as pumps, which can be prone to overheating. If you want the best temperature measurement when using this device, they’re best combined with a digital probe thermometer so you can take both surface temperature and deep water temperature readings.