Aquarium Heater Size Chart NZ: Watts per Litre, Placement & Temps

The quick answer: you need an aquarium heater if the fish you keep require water warmer than your room can reliably provide. For many tropical fish, Merck Veterinary Manual says water is kept near 25 degrees C. Cold-water fish such as many goldfish do not automatically need a heater, but every tank still needs stable, species-appropriate temperature and a separate thermometer.

Heater or no heater?

SPCA New Zealand says fish tanks must maintain species-appropriate water quality, lighting and temperature. That means the fish list decides the heater, not the box on the shelf.

For first-tank setup, read How to Set Up a First Aquarium NZ.

What to look for in a heater

• Adjustable thermostat rather than a fixed mystery setting.
• Wattage suited to tank volume, room temperature and target temperature.
• Clear maker instructions for minimum water line and placement.
• Heater guard if fish are large, boisterous or likely to knock gear.
• Indicator light so you can see when it is heating.
• Good water flow around the heater.

Do not choose heater size from tank litres alone. A small tank in a cold South Island sleepout is a different job from the same tank in a warm Auckland lounge with a heat pump.

Heater size chart (litres to watts)

Use this as a starting point, then adjust for your room. The common rule of thumb is about 1 watt per litre when the room sits within about 5 °C of your target temperature, and closer to 2 watts per litre for cold rooms — Aquarium Co-Op's heater sizing guide works on a similar basis (about 5 W per US gallon for a 10 °F lift, with a lid on).

For tanks needing 200 W or more, two smaller heaters beat one big one: heat spreads more evenly, and if one fails or sticks on, the other limits the damage. In an unheated NZ room, measure the coldest overnight temperature before you buy — the gap between room and target temperature decides the wattage, not tank litres alone.

Use a separate thermometer

Merck Veterinary Manual recommends a separate thermometer because maintaining the right temperature range is critical to fish health. Do not rely only on the number printed on the heater dial. Check the thermometer daily when the tank is new, after water changes, during cold snaps, and after moving the tank.

Water quality still matters alongside temperature. See How to Cycle a New Fish Tank NZ and Aquarium Filter Types NZ.

Placement and safety

Place the heater where water moves past it, usually near filter flow, and follow the maker's angle and submersion instructions. Unplug heaters before removing them from water or doing maintenance if the instructions require it. Let hot glass heaters cool before exposing them to air.

Avoid putting tanks in direct sun, beside a fireplace, on a cold windowsill, or in a garage where winter nights and summer days swing hard. NZ homes can have big day-night temperature changes, especially in older rentals and unheated rooms.

Goldfish and cold-water tanks

Goldfish are often described as cold-water fish, but "cold-water" does not mean a tiny bowl, sudden winter drops or summer overheating. They still need enough water volume, filtration and stable conditions. For that topic, read Do Goldfish Need a Heater in NZ? and Beginner Aquarium NZ.

When to ask an aquatics specialist

Ask before mixing species with different temperature needs. A tank that is perfect for one fish may stress another. Also ask if you keep shrimp, delicate tropical species, marine fish, outdoor ponds, or a very small desktop tank where temperature can swing quickly.

For beginner species choices, see What Fish Are Good for Beginners NZ.

Shop related categories at PetMall

Looking for aquarium heating and fishkeeping basics in New Zealand? Browse the PetMall fish range for current options and nationwide delivery.

-> Browse Fish & Aquarium Supplies

Related reading

• How to Set Up a First Aquarium NZ
• How to Cycle a New Fish Tank NZ
• Do Goldfish Need a Heater in NZ?
• Aquarium Filter Types NZ
• What Fish Are Good for Beginners NZ

References

• Aquarium Co-Op, How to Choose the Right Aquarium Heater — wattage sizing and two-heater redundancy, checked 2026-07-17: https://www.aquariumcoop.com/blogs/aquarium/how-to-choose-the-right-aquarium-heater

• SPCA New Zealand, caring for fish, checked 2026-06-05: https://www.spca.nz/advice-and-welfare/article/caring-for-fish
• SPCA New Zealand, fish welfare position statement, checked 2026-06-05: https://www.spca.nz/advice-and-welfare/article/fish
• Merck Veterinary Manual, special considerations for fish, checked 2026-06-05: https://www.merckvetmanual.com/all-other-pets/fish/special-considerations-for-fish
• Merck Veterinary Manual, providing a home for fish, checked 2026-06-05: https://www.merckvetmanual.com/all-other-pets/fish/providing-a-home-for-fish

Important notice

*General fishkeeping information for NZ owners. Match temperature to the exact species you keep, and ask an aquatics specialist or fish-savvy vet if fish are gasping, lethargic, clamped, injured or dying.*