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Most species of animal or plants we want to keep in an aquarium need a particular water temperature in order for them to remain healthy. Some need cold water kept below a certain temperature, some need a constant higher temperature than the average living room.

We use a device called the thermometer to constant monitor the water temperature to ensure the animal natural temperature is maintained.


These are typically one of three types.

1. Flat strips of a LCD temperature sensitive materials which is stuck on the outside of the aquarium and informs you of the temperature of the outside surface of the aquarium. These are not very accurate or quick to adjust, but are cheap.

2. Thin tubes of glass which you place inside the aquarium to accurately measure the water temperature. These devices nowadays contain a tiny amount of alcohol (usually blue or red in colour). These types containing alcohol are harmless if the vial breaks. Very old ones may contain mercury (typically silver in colour) which is poisonous to your fish and to you if the vial breaks.

3. Electronic versions. these usually comes with a plastic probe on the end of a length of wire which you dip into the water. The electronic unit in this case remains outside the aquarium. There are small versions of these units without wired probes that get submersed into the water and stick on the inside of the aquarium facing outwards. Another handy advantage is that some types can record the lowest or highest obtained temperature over time and some can be set to sound out an alarm if the water temperature gets too cold or too hot.

Typical use

When placing a new fish into a new tank, it is vital to match the water temperature of the bag it came in to the aquarium water. Failure to allow the fish to naturally become accustomed to the new tank temperature will cause serious shock to the animal and may kill it. First you need to know the fish's normal 'in the wild' temperature range and what the bag temperature is. Then match the new tank temperature to within a few degrees.

It is usual to match the transport bag temperature with the tank by placing the unopened bag into the tank and leaving it there for about 30 minutes to allow the bag temperature to naturally adjust to the tank temperature before opening the bag and letting the fish out.

  • Note you also need to match the pH of the two containers of water within ~0.5 pH so to prevent pH shock.