Aquarium Heaters for Winter nights

Fish are cold blooded beings, or at least thats what they taught me at school. But they can’t survive at cold temperatures inside the aquarium tank on winter nights. They need temperature monitors and heaters too. I have installed a device that pumps in hot air into the tank which keeps the temperature good for the fish, but still this isn’t good enough. I should be getting a heater soon that sells for about $20 to give it optimal conditions for survival.