Why sizing matters more than brand
A heater's job is simple — hold your tank at a steady tropical temperature, usually somewhere around 24–26°C — but getting the wattage wrong makes it hard. Too little power and the heater runs constantly yet never quite catches up on a cold night. Too much power in a small tank and a stuck thermostat can cook the water alarmingly fast. Right-sizing is the single biggest decision, and it comes down to your tank volume and how warm the room is.
If you have not chosen a tank yet, our aquariums hub covers volumes, and the heaters hub compares specific models by accuracy and build.
The watts-per-litre rule
The quick, reliable guide is around 1 watt per litre in a heated room, and closer to 1.5 W/L in a cold or unheated space. Work from your true water volume (a "60L" tank holds less once substrate and hardscape are in), then round up to the nearest available wattage:
- Up to ~60L: 50–75W in a warm room, 100W if it is cold.
- 60–120L: 100W, or 150W for a chilly room.
- 120–200L: 150–200W.
- Over 200L: split the load across two heaters (see below).
For nanos, an oversized heater is genuinely dangerous, so match the wattage carefully — our best nano heater picks are sized for small volumes. For everything else, the best aquarium heater round-up sizes each model by tank.
The safety features that actually matter
Heaters are the most common cause of tank disasters, so this is not the place to save a few pounds. Prioritise:
- Shatter-resistant body. A cracked glass heater is an electrical hazard. Modern shatter-resistant models tolerate knocks and the occasional dry run far better.
- Auto shut-off. This cuts power if the heater overheats or is lifted out of the water while running — the two situations that break cheap heaters.
- An external or clearly marked thermostat so you can set and read the target temperature precisely.
- A separate thermometer. Never trust the heater's own dial alone; a cheap standalone thermometer is your early-warning system.
Why two heaters beat one on a big tank
On tanks over roughly 200L, splitting the wattage across two smaller heaters is the professional move. It buys you redundancy against the two failure modes that kill fish. If a thermostat sticks on, a single half-power heater can only raise the temperature slowly, giving you time to notice before it becomes lethal. If one fails off, the second heater keeps the tank from plunging overnight. It also spreads heat more evenly through a long tank.
Setting up and settling in
Position the heater near the filter outflow so moving water carries warmth around the tank, and mount it at an angle or horizontally low down if the maker allows. Once it is running, check the temperature over a few days with your independent thermometer and adjust the dial until it holds steady. From there it is almost maintenance-free — just wipe algae off the tube occasionally and keep half an eye on the reading during your weekly maintenance routine. If you are still planning the wider build, the tank set-up guide walks through where the heater fits in the order of operations.