Do ranchu need a heater?
Usually not. Ranchu are cold-water fish. What they cannot handle is rapid change, and that is where a heater sometimes earns its place.
The comfortable range
Ranchu do well from 65 to 75 °F (18 to 24 °C). They tolerate cooler water down to the 50s if the change is gradual, and breeders overwinter them outdoors in many climates. They do not need tropical temperatures and they suffer above about 80 °F, where oxygen drops and bacteria multiply.
When a heater helps
- A cold room in winter. If the tank drops below 60 °F, digestion slows and floating becomes more common. A heater set as a floor at 65 °F keeps the fish eating.
- Daily swings. A room that is 62 °F at night and 75 °F by afternoon stresses fish more than either temperature alone. A heater removes the low end of the swing.
- Growing young fish. Tosai grow faster and put on wen in the low 70s. See wen growth.
- Treating disease. Ich treatment works faster at 78 °F. Raise slowly, treat, then lower slowly.
Setting it up
Choose a heater rated for your volume, set it at 65 to 68 °F rather than a tropical setting, and use a heater guard: ranchu rest against warm objects and can burn their wen. Add a thermometer at the far end of the tank so you see what the fish sees, not what the heater dial claims. During water changes, match the new water to the tank within a degree or two.