Yes, kuhli loaches need a heater. Kuhli loaches are tropical fish that originate from freshwater streams in Southeast Asia. They need warm temperatures of around 73–86°F (22.7°–30°C) to thrive.
If the water is too cold or too hot, kuhli loaches may experience growth stunts, stress, illnesses, and even death.
Even if their tank is located in a warm area, the water temperature can still fluctuate throughout the day, so you still need to install a heater in their tank.
You should install a heater and a thermometer to help you monitor the temperature throughout the day. Make sure to pick the right heater according to the tank size. The bigger the tank, the higher the heater’s wattage should be.