The short answer
Yes — guppies need a heater in most homes. Guppies are tropical fish that do best in stable water around 24 °C (roughly 22–26 °C). While they’re hardy and tolerate a range, they suffer in cold or fluctuating water: a chilled guppy is more prone to disease, sluggish and shorter-lived. Unless your room reliably stays warm year-round, a thermostatically controlled heater is the safe choice.
Why guppies need warmth
Being tropical, guppies rely on warm water for a healthy metabolism and immune system. Keep them too cool and you’ll often see:
- Lethargy and reduced appetite
- Greater susceptibility to disease, including ich
- Shorter lifespans overall
Just as important as the temperature itself is stability. Room temperatures swing between day and night and across seasons; a heater with a thermostat holds the water steady, which guppies handle far better than constant fluctuation.
When you might get away without one
If you live somewhere genuinely warm and your room reliably stays in the low-to-mid 20s °C day and night, year-round, guppies can manage without a heater. But that’s uncommon in most homes, and the risk of a cold snap stressing the fish makes a heater cheap insurance. When in doubt, heat the tank.
The bigger picture
A heater is one of the basics for keeping guppies well. Read the full guppy care guide for tank size, water and tank mates, see what to expect in how long do guppies live, and keep conditions stable with regular maintenance. To confirm your target temperature, see what temperature should a tropical aquarium be.