An essential accessory for an aquarium heater is a thermometer so that you can keep a check on the water temperature which will tell you if the heater is working or if an adjustment is required.
Both Tropical Freshwater & Marine aquariums need to be maintained at a temperature of 24 to 26C. Some fish species such as Discus require a water temperature somewhat higher. The easiest way to achieve the required temperature is with a combined heater & thermostat device. These attach to the aquarium glass with suckers & are adjustable so that exactly the correct temperature can be set.
Aquarium Heaters are available in a range of wattages which give a greater or lesser heat output. A particular wattage is suitable for a given volume (litres) of water. (see product pages for more information & guidelines).