A good quality heater is a requirement for any aquarium salt water or fresh water. The temperature should be kept stable so using the right size heater for your aquarium water volume is important. If you try to use a heater with too small a wattage for your aquarium it will battle to keep the temperature stable. If you live in a very cold area or your house tends to drop to low temperatures when the cold weather is here you can always go up a size wattage in your heater. If your aquarium is large and he temperature still dropping you may need to use 2 heater . Most fresh water aquariums generally would run between 24 to 26 degrees celcius. With discus aquariums some people will run the temperature slightly higher.
Thermometers go hand in hand with your heater, you do not know what your temperature is without them.
25 watt heater———–10 ltrs to 20ltrs
55 watt heater———–40 ltrs to 50 ltrs
100 watt heater———-50 ltrs to 70 ltrs
150 watt heater———-70 ltrs to 130 ltrs
200 watt heater———-130 ltrs to 200 ltrs
300 watt heater———–200 ltrs to 300 ltrs