The ideal temperature for tropical fish depends on the species, but in general, they are healthiest in the range of 75-80°F (24-27°C) . Most aquarium heaters are preset to 78°F (25°C), which is the temperature that an average tropical aquarium should be. However, it is important to research the specific temperature requirements of the species of fish in the aquarium, as some may require slightly higher or lower temperatures to remain healthy. It is recommended to use a thermometer to monitor the temperature of the water and ensure it stays within the appropriate range. It is also important to avoid placing the fish tank near a source of heat or air conditioning, as this can cause fluctuations in temperature.