The number your heater is not telling you.
The dial on a heater is a setting, not a reading. A separate thermometer is how you find out that a thermostat has drifted, and drift is what kills tanks quietly.
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Colour: Black
Why you'll love it
Cheap heater thermostats drift, and the dial keeps saying what you set it to regardless. An independent thermometer is the only way to know what the water is actually doing.
Rated at ±1 °C, so treat the absolute figure as approximate. What matters is that the same instrument reads the same each morning: a two degree slide is the warning, whatever the true value.
Stick on strip thermometers fade and in tank glass ones break. Probe in, display out means the electronics never sit in the water.
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A heater's dial tells you what it has been asked to do, not what the water is actually doing. Thermostats drift with age, and a heater set to 25 degrees can be holding 22 or 28 after a year without anything looking wrong from the outside.
The failure that matters most is a thermostat sticking closed. The heater then runs continuously, and a small tank can climb several degrees in a few hours. It is one of the more common ways a tank is lost, and it is entirely preventable by looking at an independent reading once a day.
Fish are far more sensitive to change than to any particular number within their range. A tank steady at 24 degrees is better than one averaging 25 while swinging two degrees either side, because it is the swing that stresses fish and suppresses their immune response.
That is why resolution to a tenth of a degree is useful even though the accuracy is about a degree. You are watching for movement rather than for an absolute figure: a reading that has changed by half a degree since yesterday tells you something even if the absolute number is slightly off.
Put the probe at the opposite end of the tank from the heater, at mid depth. Next to the heater it reads the warm plume rather than the tank, and right at the surface it reads a layer that is cooler than the body of water. The far end at mid depth gives the figure that matters.
Keep it out of the direct outflow of a filter for the same reason. The display mounts outside on the glass, which keeps the electronics away from the water entirely and means you can read it without opening the lid, and the probe can stay in permanently.
In the box
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The details
Algae grows on the probe like anything else in the tank, and a thick coating slows its response. Wipe it during your weekly water change, and check the lead where it passes over the rim, since that is where a cable is pinched by a lid and eventually fails.
A button cell lasts a long time but it does die, and a dead thermometer is a thermometer telling you nothing at all. Change it as soon as the display starts to dim rather than waiting, and check the reading against a second thermometer occasionally to confirm it has not wandered.
Loved by real pet parents
The dial on our heater said one thing and this probe said something two degrees lower, which explains a few things. Reading to a tenth of a degree means I can watch the water settle after a change rather than guessing. The display sticks to the side panel and stays dry.
It takes a second to glance at on my way past, and that has become part of the routine along with checking the filter flow. The point is not the exact number, it is that the same number comes up each day. When it does not, I know to go and look at the heater.
The probe lead means the display has to sit fairly close to where the probe goes in, so I could not put it on the front of the cabinet as planned. It reads well and the battery was already in it. Just plan where it will live before you stick the pad down.
It was reading within a minute of being unpacked, with no fiddling about with a tiny cell and a screwdriver. I stuck the white housing on the side of the tank where it disappears against the wall. Nine weeks on and it is still running on the original battery.
Over about a fortnight the morning reading crept down by nearly two degrees, which I would never have noticed from the heater dial. I swapped the heater and it has been steady since. That is exactly what I bought it for.
I have a glass thermometer in the same tank and the two disagree by about a degree, which the description does admit is within the stated tolerance. It is consistent with itself so I still use it for spotting changes. I would not rely on the absolute figure though.
The third glass thermometer I broke was the last one. Having the electronics outside the tank and only the probe in the water has solved that entirely, and there is no faded stick on strip to squint at either.
The probe would not stay where I put it and kept drifting up towards the surface, so I ended up tucking the lead behind the filter pipe to hold it down. Once it is wedged in place it has been steady and the readings look sensible. A better clip would make this a five.
I chose black to match the tank trim and you genuinely do not notice it from the front of the room. The screen is small at about five centimetres across, but the digits are clear from a metre away, which is as close as I ever stand.
Good questions
About ±1 °C, which is normal at this price. It is reliable for spotting change rather than for calibrating equipment. If you need a precise absolute figure, check it once against a lab thermometer and note the offset.
Yes. The probe and its lead are made to sit in the water; only the display needs to stay dry. Run the lead over the rim rather than pinching it under a lid.
At the opposite end of the tank from the heater, and not right beside the filter outlet. Next to the heater it reads the heater, not the tank.
Around a degree in absolute terms, with resolution to a tenth. That is fine for aquarium use, since you are mostly watching for change rather than needing a precise figure.
Yes, that is how it is meant to be used. The electronics are outside the tank and only the sealed probe sits in the water.
At the opposite end from the heater, at mid depth, and out of the filter outflow. Beside the heater it reads the warm plume rather than the tank.
Last reviewed 30 August 2026
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