Bristlenose plecos need a temperature of approximately 74°F to 80°F (23°C to 26.5°C). Endemic to South America, these bottom-dwelling creatures require warmth to thrive.
If the water is too cold, especially once it gets below 60°F (15.5°C), bristlenose plecos will become weak, sick, and stressed. Eventually, they will die, either because of the cold or their weakened immune system.
Because bristlenose plecos require such warm water, you need to install a heater in their tank. An adjustable heater with a built-in thermometer is best so that you can keep the water temperature stable throughout the seasons.