The short answer
It depends on the fish. True cold-water species don’t need a heater, but tropical fish do. White cloud minnows, medaka and goldfish are happy at room temperature, while guppies, tetras, bettas and most popular aquarium fish need the steady warmth of a heater to stay healthy.
When you can skip the heater
If you choose genuine cold-water fish, an unheated tank is fine — even ideal. Good options include:
- White cloud mountain minnow — the go-to unheated nano fish.
- Medaka ricefish — hardy and comfortable in cool water.
- Goldfish — cold-water, but only in a large, well-filtered tank.
See what fish can live in cold water for the full list.
Why tropical fish need one
Tropical fish evolved in warm, stable waters, typically 24–27 °C. The issue isn’t only that rooms are often too cool — it’s that they swing, dropping at night and in winter. Those fluctuations stress fish, suppress their immune systems and invite disease. A thermostatically controlled heater holds a steady temperature, which is exactly what keeps tropical fish thriving.
Choosing a heater
If you’re keeping tropical fish, match the heater wattage to your tank volume and pick one with a reliable thermostat. Browse aquarium heaters and see what size heater do I need. Deciding between a cold-water or tropical tank? Our beginner aquarium guide can help you pick a direction.