Marina Submersible Pre-Set Heater 300w

Heater wattage is not a preference, it is arithmetic: roughly a watt per litre for a room at normal temperature, more if the tank sits somewhere cold. At 300 watts this one is matched to aquariums up to 300 litres, and putting it in something much smaller is as wrong as putting it in something much bigger.

It is a submersible heater 27cm long, in impact-resistant glass with waterproof sealing, and it works in fresh or salt water. The temperature dial is on the outside where you can reach it, the settings are printed to be readable through water, and there is an on/off pilot light so you can tell at a glance whether it is calling for heat. A bumper guard keeps fish off the glass, a bracket with suction cups holds it in place, and the electrical cord is 1.8 metres. The heating range runs from 20°C to 28°C, and it is CSA approved.

• 300 watts, for aquariums up to 300 litres
• 27cm long
• Fresh or salt water
• Temperature range 20°C to 28°C
• On/off pilot light and an easy-to-reach set dial
• Bumper guard, suction-cup bracket, 1.8m cord

Match the wattage to the tank, and never run it dry. A 300W heater in a small tank will overshoot before the thermostat catches it, so size it to the water rather than to the price. And the rule that matters most with any submersible heater: unplug it and let it cool before a water change that drops the level below it — glass heaters crack when they are switched on in air, and that is the failure that kills a tank. A separate thermometer is worth having too; a heater's own dial tells you what it is asking for, not what the water is doing.

Who it's for: anyone running a tropical tank up to 300 litres, fresh or marine, who wants a heater with a visible pilot light rather than a silent one.