Pond thermometers

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Last updated Oct 2014

Fishkeepers, especially Koi keepers, should check their pond temperature regularly, in order to track water conditions and trends, and care for the creatures accordingly. So a thermometer is an essential piece of equipment, readily available from water gardener suppliers such as Bradshaws Direct and Pondkeeper, or swimming pool suppliers.

Tip: Start a log book and be sure to record your readings regularly to build up a picture of your pond’s condition throughout the year.

Too hot or cold?
The right depth for testing
Pond thermometer types – floating/columnar
Pond thermometer types – sensor on cable
Pond thermometer types – infra-red
Buying guide – checklist

Links for thermometers:

Too hot or cold?

Koi prefer temperatures in the 8-28°C (48-84°F) range – fortunately, you can safely let the pond water drop to 4.5°C (40°F) without harm. It is, however, best to keep the maximum to 21°C (70°F) if you can, for various reasons, including the fact that Koi colour can fade at higher temperatures. As the water warms up, there’s also increased activity, so there’s an increasing need for food (which means more waste for the filters to deal with) and more importantly, oxygen. Because warmer water also holds less oxygen, aeration becomes doubly essential as temperatures soar.

Equally important to monitor is the fluctuation in conditions as Koi dislike large swings in temperature over short periods. Ideally, you should take your pond’s temperature at regular intervals to check for stability, and take action if necessary.

When you see temperatures warming up in spring or cooling down in autumn, you’ll know that it’s time to modify feeding. Koi can digest animal protein at higher temperatures, but at around 10°C (50°F), they are better able to digest vegetable proteins. A degree or so less and they can’t digest any food at all and will ignore it. At this point, your fish are probably a better indicator of feeding routine than your thermometer is.

You’ll also certainly find a thermometer useful if you need to apply medication within a recommended temperature range. And for transferring fish, or transporting them, it’s important to know the temperature that they’ve been used to and the amount of equalisation required.

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The right depth for testing

Unlike the ground, water is quick to heat up and cool down, so you won’t find much difference at different depths, other than at the surface where low temperatures or calm sunny spells will make a noticeable difference. Take a surface reading by all means, but remember to take one about 5cm (2in) down too for a more accurate guide, with deeper readings (about half-way down) to check that conditions are remaining constant.

You may find similar temperatures at different depths, or very different ones, for various reasons.

One peculiarity of water is that it’s denser at 4°C than at lower temperatures, so when water at the surface cools to this level, it sinks to the bottom of the pond to form a relatively warm pocket beneath colder water and ice. This is how wildlife survives in a frozen pond, providing it’s deep enough for ice to form before all the water becomes chilled. However, you won’t benefit from this effect if you keep the pump running and bottom drains functioning through winter, because the water will be continually mixed. Similarly, if the fish are active, they also stir up the water and equalise temperatures.

A partly sunken pond benefits from the stabilising influence of the surrounding soil, and the recommended depth for a Koi pool is around 1.2m (4ft) to minimise temperature fluctuations. These deep ponds stay cool in the heat of summer and are unlikely to freeze solid in the average UK winter. With an insulating cover on top, you might not even have ice on the surface, except in the worst of winters.

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Pond thermometer types – floating

Traditional styles are still available, with a reservoir of liquid that expands with heat and rises up a glass capillary tube alongside a calibrated scale. The liquid is no longer mercury, which has been banned for its toxicity, and is now usually coloured alcohol. Less easy to break are those with a sensor and digital readout.

Both types are readily available as floating thermometers, enabling you to make useful spot checks, but you will need to tether them so you can reel them in to make a reading. Traditional types include those from Bradshaws Direct or from Pondkeeper including PondXtra’s that is suspended below an ornamental frog float. Both suppliers have digital versions: battery-powered examples are made by Superfish and Velda; there are also solar-powered versions from Superfish and Velda. These simple thermometers are robust and easy to read, but you need to add weights to the unit if you want to discover deep-down temperatures, and the display part of digital versions may not like immersion for long – so check the instructions!

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Pond thermometer types – digital with a cable+sensor

Digital thermometers register temperature differences with electronic wizardry (measuring differences in electrical resistance) and produce an LCD read out are also available. These are battery- or mains-operated and increasingly common, but you must keep circuitry dry to avoid damage to it, you and the fish. A weatherproof box could be useful.

For peace of mind, a thermometer that has an alert feature is worth searching out. Then, depending on the season, you can set an alarm if the temperature falls to danger levels (indicating a heater failure) or rises to a point where you need to address oxygenation. One such dual display inside/outside thermometer is available from Maplin.

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Pond thermometer types – infra-red

There’s a new breed of thermometer that is contact-free – the Maplin type that you just point at the item you are measuring. Although not designed with pools in mind, this type might suit you.

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Buying guide – checklist

Having tested various thermometers, this is what we’d like to see:

  • Easy-to-read figures in °C or °F
  • Stated level of accuracy (eg accurate to 1°C or 0.5°C)
  • Range from 0-30+°C (32-86+°F)
  • Quick and easy to use
  • Robust unit designed for frequent handling
  • Records maximum and minimum temperatures with easy reset
  • Possibility of permanent installation wireless so not limited by length of lead to the probe – temp sensor/probe wires cannot usually be extended
  • For fishkeepers – an alarm that alerts you to an unwanted temp (either high or low)
  • For electronic thermometers – the whole unit is weatherproof so there is no need to protect it from the wet

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