The short answer
For tropical fish, yes — you need a heater. The vast majority of aquarium fish (tetras, guppies, bettas, corydoras, most cichlids) come from warm waters and need a stable temperature, typically 24–26°C. A heater doesn’t just warm the water; it keeps it steady, and stability is what keeps fish healthy. If you’re keeping coldwater species like goldfish, a heater is optional.
Why stability matters more than warmth
Fish are cold-blooded, so their body temperature follows the water. Sudden drops — an overnight chill, a cold snap — stress their immune system and can trigger disease like ich. A thermostatic heater switches on and off automatically to hold a constant temperature, ironing out the daily swings a room naturally has.
Even a “warm” living room cools by several degrees overnight. That fluctuation is exactly what a heater prevents.
When you can skip a heater
- Coldwater species: goldfish, white cloud mountain minnows and some others prefer cooler water and don’t need heating.
- Very stable warm homes: if your room genuinely never drops below the low 20s year-round, some hardy species cope — but this is rare and risky.
For almost everyone else, a heater is essential kit. It’s inexpensive and prevents a lot of heartache.
Choosing and sizing a heater
Pick a heater rated for your tank volume — as a rule of thumb, around 1 watt per litre (more in cold rooms). Undersized heaters run constantly and struggle; oversized ones can overshoot.
See the aquarium heaters hub for the full range, or jump to our best aquarium heater picks. For exact wattage, read what size heater do I need. You may also want a lid to reduce heat loss from evaporation.