The short answer
Before assuming your heater is dead, run three quick checks: the indicator light, the thermostat setting, and an independent thermometer. Most “broken heater” panics turn out to be a dial nudged too low, a heater in still water, or a perfectly fine unit that had simply reached temperature and switched itself off.
Work through the checklist
- Indicator light. Most heaters glow when actively heating and go dark once the set temperature is reached. A dark light can mean the tank is already warm — not a fault. A light that never comes on in a cold tank points to a failed element or thermostat.
- Thermostat setting. Check the dial hasn’t been knocked during a water change. Turn it up a couple of degrees and see if the light responds.
- Cross-check with a thermometer. Hang a separate thermometer at the far end of the tank. This tells you the actual temperature, independent of the heater’s own reading.
- Water flow. A heater in a dead corner heats a pocket of water the thermostat never senses. Make sure it sits near filter flow.
When it’s genuinely dead
If the light never comes on with a cold tank, the thermostat clicks but nothing warms, or the glass is cracked or fogged inside, the heater has failed and needs replacing. Heaters are consumables — most last three to five years. Keep a spare if your fish are sensitive to cold.
To choose a dependable replacement, see our best aquarium heater picks and the how to choose an aquarium heater guide, or browse all aquarium heaters. Related: what size heater do I need? and is my aquarium too hot?