How to Keep Your Betta Warm in Winter
Table of contents
Why winter is risky for bettas
Bettas are tropical fish and need warm water year-round. In winter, room temperatures drop, tanks lose heat through the glass and lid, and windowsill tanks can swing wildly between day and night. If the water falls below about 74°F, your betta's metabolism slows: it becomes sluggish, stops eating, and is far more prone to disease.
Is your heater strong enough?
A heater that was fine in summer may struggle in a cold room. For winter, aim higher on wattage:
| Tank size | Winter wattage |
|---|---|
| 5 gallons | 25–50 W (use 50 W if the room gets cold) |
| 10 gallons | 50–75 W |
| 20 gallons | 100–150 W |
A rough winter rule is about 5 watts per gallon, more if the room is chilly. Choose an adjustable heater so you can dial in 78–80°F — see our guide on choosing a betta tank heater.
Position the tank wisely
- Away from windows, doors, and drafts — the biggest source of cold and temperature swings.
- Not directly under air-conditioning or heat vents — rapid swings stress the fish.
- Not in direct sunlight — it can overheat the tank in the day, then leave it cold at night.
- Keep the lid on and the water level high enough that the heater stays fully submerged.
Check the temperature daily
Heater dials drift, and a cold room makes small mistakes matter more. Use a separate thermometer and read it once a day — in the morning, when the room is coldest. If it's below 78°F, turn the heater up a little and re-check in a few hours. Avoid large, sudden changes; aim for steady.
Cold-water red flags
If your betta starts lying on the bottom, looks sluggish, pale, or won't eat, cold water is a likely cause. Warm the tank back to 78–80°F gradually, then watch. Persistent symptoms could mean illness — see our disease guide.
Related guides: Does a betta need a heater? · Water change routine · Why is my betta on the bottom?