The majority of freshwater tropical fish like to live in water which is around 75F (24C), so unless you live in a climate where the water will stay at this temperature, a heater is required. The majority of heaters available for aquariums are glass tubes, which have a built in thermostat, to ensure a constant water temperature. It is important to buy the correct size heater for the aquarium. Too large, and it will heat up the water too quickly, which can be fatal to fish. Too small, and the heater will need to work too hard to maintain the required temperature, and will eventually burn out and fail.
As a rule of thumb, allow 10 watts per gallon (3.8 litres) of water, when choosing a heater. If for any reason there is a power cut, do not panic. It takes several hours for the temperature of the water to drop significantly, so unless it is especially cold and the power supply is off for a long time, no action is required.
It is also advisable to purchase a small thermometer which can be fixed inside the aquarium in a convenient place, so the temperature can be monitored easily.