JBL Float suction-cup aquarium thermometer

The thermometer works by simple expansion: when the water warms up, the liquid in the column expands and its level rises on the scale, with no battery, no screen and no adjustment. A ballast of beads at the foot of the tube keeps it upright when you detach it from the glass, so it can also float freely in a tank without a smooth wall. The suction cup withstands acidic and alkaline water alike, and the complete thermometer weighs 17 g.

The scale covers everything you encounter in a freshwater tank, from the cool water of an indoor pond in winter to a heated tropical aquarium. It also carries a colour marker: a green band around 25 °C, flanked by two yellow bands at 20 and 30 °C, which shows the usual range of a tropical aquarium. So you can see at a glance whether a heated tank strays from it, while the figure, read to the nearest degree, lets you track the changes from one day to the next.

Placement changes the reading. Stick the thermometer opposite the heater, at mid-height, in the coldest zone of the tank: this is the zone that shows whether the whole volume reaches the target temperature, whereas a reading taken right next to the heating element always shows too high a value. Also avoid the glass panel exposed to direct sunlight, which warms the column itself and makes you read the temperature of the glass rather than that of the water. Finally, wipe the glass with a cloth before fixing the suction cup, so that it sticks durably.