Heating
Aquarium heater
Heaters fail in both directions and both failures are dangerous. The two practical safeguards are an independent thermometer and, in larger tanks, two smaller heaters instead of one large one so that a single failure cannot overheat the tank.
Short answer
A submersible heating element with a thermostat. Its dial is not to be trusted; a separate thermometer is what tells you the actual temperature.
What it does
- Maintains water temperature against room temperature
Strengths and where it fits
Strengths
- Inexpensive relative to what it protects
- Widely available in every size
Good fit for
- Any tropical tank
Limitations and where it is the wrong choice
Limitations
- Thermostat calibration is often several degrees out
- Failure modes include stuck-on, which can cook a tank in hours
- Glass heaters can crack if exposed to air while powered
- Placement matters — a heater in still water reads the wrong temperature
Poor fit for
- Genuinely temperate species tanks, where a heater is often unnecessary or harmful
Sizing
- A common working figure is 1–1.5 W per litre for a tank in a heated room, rising for a cold room or a large temperature lift. AquariumHQ's heater sizing tool shows the calculation and its assumptions.
- In tanks above about 150 L, two heaters of half the wattage each are safer than one.
- Position in flow, near the filter intake or outflow.