When people ask “what temperature should I keep guppies at?”, they often expect one magic number. In reality, guppies do well across a range — but they do poorly with swings. Daily fluctuations stress fish, dull colour, and make disease more likely. A heater is less about warming the tank and more about keeping the temperature steady.
A stable range that works well
For most home tanks, 24–26°C is a comfortable sweet spot. It supports appetite, keeps behaviour active, and helps females recover well after drops. Running colder can slow metabolism and immunity; running hotter increases oxygen demand and can shorten lifespan if consistently high.
Why stability beats “perfect”
A tank that sits at 24.5°C day and night is usually healthier than a tank that bounces between 21°C overnight and 26°C in the afternoon. Fish feel the swing, even if your test results look fine. Many “random” fin clamping episodes and stress behaviours trace back to this daily cycle.
Common heater mistakes
- Setting it too high: warm water holds less oxygen; fish breathe faster and stress rises.
- Placing the heater in dead flow: heat doesn’t spread evenly; you get hot/cold zones.
- No thermometer check: heater dials are not always accurate. A basic thermometer confirms the real number.
Best practice setup
- place heater near filter flow so warmed water circulates
- keep temperature changes slow (adjust no more than ~1°C per day)
- increase aeration during warm spells or if fish breathe faster
What to do if your room runs hot or cold
If your room regularly gets very warm, focus on oxygen first: surface agitation and sensible feeding help. If your room is cold overnight, a heater prevents the nightly drop that often triggers stress.
A stable temperature is one of the easiest wins in guppy keeping. It improves colour, appetite, and long-term resilience — and makes everything else (feeding, breeding, maintenance) more predictable.
