Fish will prefer an aquarium background because there are a few benefits to having an aquarium background. If you have an aquarium background on two to three sides of your tank, then it helps reduce the chances that your fish will see its reflection. This is important because if your fish sees its reflection, it may assume that there is another fish in the tank and try to fight with it, which can make them stressed. Besides, the background of the tank also helps to hide the cords and tubes of your aquarium filter and heater. Also, an aquarium background can enhance the look of your fish tank.