Where the Myth Comes From
Wild guppies survive in some cooler waters in their native range, and they tolerate temperatures as low as 65°F for short periods. This is a survival trait, not a preference.
Stores selling "cold water guppies" want to skip the cost of heaters in display tanks. The fish look fine for months but lifespan drops by 50-70%.
Cold Water Symptoms
Slow movement, faded color, refusal to breed, susceptibility to disease, and shortened lifespan (1 year vs 2-3 normal).
In water below 70°F, ich and columnaris become much more common. The fish immune system simply cannot cope.
- ✦Use a thermometer — never trust the heater dial.
- ✦Even ambient room temperature varies day to night.
The Right Temperature
78°F is ideal. Range: 75-80°F. A 50W heater costs $15 and pays for itself in fish you do not lose.
In Cambodia, room temperature in air-conditioned spaces drops to 72°F at night — a heater is needed even in tropical climates.