How to Choose an Aquarium Heater Wattage by Tank Size (Winter Guide)

Brisbane winters are mild compared to the southern states, but an unheated fish room can still drop a tank from 25°C to under 20°C overnight, especially in a garage or a tiled sunroom. The generic pet shop rule of "5 watts per litre" was written for houses that never see single digit nights. If your tank sits in a cold room with tiled floors or a draughty window, that rule will leave your heater running flat out just to hold temperature, with no reserve for a genuinely cold snap in July or August.

What you need

• A heater rated for your tank volume, ideally two smaller units instead of one large one
• An accurate thermometer or controller probe, not just the heater's built in dial
• A sense of how cold your room actually gets overnight, not just the daytime reading

Winter wattage guide by tank size

Steps to size your heater correctly

1. Measure your tank's actual volume in litres, not the display size on the box, since sumps and rock displacement change the real water volume.
2. Check the coldest temperature your fish room hits overnight in winter. A cheap min/max thermometer left in the room for a week tells you more than guesswork.
3. Use the cold room column above if your tank is in a garage, tiled area, or a room with no heating or insulation.
4. Split the wattage across two heaters where possible. A single 300W unit sticking on is a bigger risk than two 150W units, and if one fails the other keeps the tank from crashing.
5. Pair the heater with a controller or a unit with an accurate built in sensor. The Apex controller range can monitor temperature and alert you if a heater sticks on or fails, which matters most for reef tanks holding coral.
6. For reef systems specifically, treat the second heater as insurance rather than an optional extra. A backup heater for reef tank setups is cheap compared to the cost of a coral collection lost to a three degree overnight drop.

Heaters worth considering

The Dymax Flexz range comes in a few sizes that suit this two heater approach nicely. The Flexz 3 sits around $90 and works well as a backup or in smaller tanks, the Flexz 5 at $100 suits mid sized setups, and the Flexz 7 at $110 handles larger volumes or acts as one half of a split pair in a 300L plus system. Running two of these rather than one oversized heater gives you redundancy without much extra cost, and if one unit fails you still have heat while you source a replacement.

Browse the full heating and cooling range for options across tank sizes, and check the controllers range if you want temperature alerts sent to your phone rather than relying on a glance at the tank each morning.

Prices mentioned are accurate at the time of publishing and are subject to change. Please check the product page for the current price.