Modern heating devices are usually reliable and accurate. However, as heat plays such a critical role in a tropical aquarium, it is worthwhile investing in a thermometer to monitor temperature.

Cheapest of the thermometers is the liquid crystal stick-on strip. Temperature is indicated by crystals changing colour next to a marked scale. These thermometers are easy to read and can be positioned anywhere on the outside of the aquarium glass but once in position they should not be removed as damage to the crystals can occur. In my experience, accuracy of liquid crystal thermometers is limited to ± 2°F and this deteriorates if the aquarium glass is thick (>10mm). Liquid crystal thermometers can only be calibrated against a more accurate thermometer but some inaccuracy can be removed when purchasing; try to pick one which indicates the same temperature as the majority of others on sale.

Digital thermometers are the most accurate (± 0.1°F) and hence expensive thermometer. They consist of a digital display, which can usually be switched between degrees Celsius and Fahrenheit, and a wire probe that can be placed discreetly anywhere inside the aquarium. They react quickly to temperature and like glass thermometers, they can be easily moved to service a number of tanks. Again they can be calibrated by taking body temperature.

Conventional glass thermometers containing alcohol are a more accurate alternative. Usually correct to ± 1°F they can easily be calibrated by taking your body temperature (98.6°F / 37°C). Sometimes these thermometers are intended to be free floating in the aquarium but invariably they always find the most inaccessible places to come to rest. Suckers or hooks are available to secure the thermometer to the side of the tank but I find that their size is obtrusive.

Similar thermometers containing mercury are available but should be avoided in the aquarium. If the glass breaks, the mercury will poison the fish.