Aquarium heaters: selection, wattage, failures

The heater is the aquarium's most underrated piece of gear. A good one works silently for 5+ years; a bad one can kill every fish in a single night — either by overheating or by failing in a cold room.

Why a heater

Tropical fish need 24–28 °C. Most rooms average 19–22 °C, lower at night. Without a heater fish survive for days, but immunity drops and disease appears — ich is the typical consequence.

Heater types

Submersible (in-tank)

A glass or plastic tube with heating element and thermostat that sits in the tank. The most popular type. Downsides: visible, and fish can get burned if they press against it while heating.

In-line (canister-mounted)

Mounts on the inlet/outlet hose of a canister filter. Invisible inside the tank, fish can't touch it. Downsides: needs a canister filter, more expensive.

Substrate heating mat

Heats the tank floor from below through the glass. Useful for breeding tanks with bottom-deposited eggs or for nudging temperature up 1–2 °C in cold flats. Not enough as the primary heater.

Wattage sizing

Rule of thumb: 1–1.5 W per litre for a 5–6 °C delta from room. Examples: • 50 L at 22 °C room, target 26 °C → 50 W. • 100 L at 18 °C room, target 25 °C → 150–200 W. • 300 L in an unheated garage → two 200 W (redundancy + even heat). Under-sizing is safer than over-sizing: a weak heater simply won't reach target, an oversized one boils the fish on failure.

Thermostat accuracy

Cheap heaters drift ±2 °C from setpoint. Quality models (Eheim, Fluval, Hydor) hold ±0.5 °C. Always verify with a second independent thermometer during the first 24 hours after installation.

Backup heater

In tanks 100 L and up — two heaters at half-wattage each. Logic: if one sticks on, the other compensates without boiling the tank. If one fails off, the other holds temperature until repair.

In nano tanks (under 30 L) backup is impractical — too little water to absorb any temperature swing.

Common failures

Stuck on (overcook)

The most dangerous failure mode. The thermostat doesn't release the relay; heat climbs until you notice. Signs: fish at the surface, swimming sideways, water warm to the touch. Overnight temperature can hit 35 °C — fatal.

Defence: backup heater + external thermometer + (optional) a thermostat plug with auto-cutoff at a high threshold.

Stuck off

Silent failure: temperature slowly drops to room. Fish survive longer (tropicals tolerate 18 °C for a week), but immunity tanks and disease follows. Defence: glance at the thermometer every day.

Coldwater tanks

Goldfish, white cloud mountain minnows, sturgeon — happy at 18–22 °C. In most rooms no heater needed. Exceptions: basements and unheated spaces below 15 °C in winter — then a heater at 18 °C floor.

Installation

• Submersible — place in a flow zone (near filter outlet or pump) for even distribution. • Never run it in air — it cracks from overheating in seconds. • Before removal — unplug 15 minutes ahead so it cools (hot glass cracks in cold water).

A good heater works silently for five years. A bad one kills everything in a single night. Skimping on this piece of gear is the most expensive mistake in fishkeeping.