Aquarium Heaters: What Wattage and Which Thermostat Should You Choose?
Most ornamental fish are tropical: they live at 24 to 27 °C. A room heated to 19 or 20 °C simply isn't enough. The aquarium heater — also called a submersible or combined heater — is therefore an essential piece of equipment. Here's how to size it and use it safely.
What wattage for what volume?
The general rule: around 1 watt per litre when the aquarium sits in a heated room. Allow 1.5 W per litre if the room is cool (a conservatory, a barely heated bedroom).
- 54 to 60-litre tank: a 50 to 75 W heater (10 to 20 €).
- 100 to 120-litre tank: a 100 to 150 W heater (15 to 30 €).
- 200 litres and up: 200 to 300 W, or two heaters at half the wattage each (25 to 45 €).
Fitting two lower-wattage units in a large tank is a popular trick: if one fails, the other limits the temperature drop; if one sticks in the on position, overheating happens more slowly.
The thermostat: built-in or external
Almost all modern heaters (Eheim, JBL, Tetra, among others) include an adjustable thermostat that cuts the heating once the set point is reached. For extra precision, external electronic thermostats exist (20 to 40 €) into which you plug the heater: they display the actual temperature and cut the power if it's exceeded. A welcome safety net for high-value tanks.
Placement and best practice
- Position the heater near the filter outlet to distribute the heat evenly.
- Submerge it fully, respecting the minimum water level indicated.
- Unplug it 15 minutes before any water change: out of the water, a hot tube can crack.
- Check the temperature with an independent thermometer (2 to 8 €), never relying solely on the heater's dial.
Special cases: bettas and goldfish
The betta is a tropical fish that demands 25 to 27 °C: a heater is mandatory, even in a nano tank. The goldfish, by contrast, is a coldwater fish that needs no heater indoors. To feed these two species with opposite needs properly, see our guide to feeding bettas and goldfish.
The key takeaway
A reliable heater costs less than 30 € and protects your entire stock: it's not the place to cut corners. Glance at the temperature every day, and keep a spare heater on hand if you can. Find our other equipment guides in the accessories section and all our advice on the Planète Pets fish hub.
This guide is part of Planète Pets’s Fishkeeping universe. Our advice is general in nature: for any health concern, your veterinarian remains the only reference.