Alan always turns the aquarium light on before putting fish food into the tank. after a while, he notices that the fish swim to the top to look for the food as soon as he turns on the light. in this example, the is the unconditioned stimulus.

This situation is a conditioned stimulus and not an unconditioned one.

The fish here associated between the light being turned on and the food being provided. It established a relation between the two where it realized that whenever the light is turned on, this meant that food was about to be served.

Based on this, the answer would be:
Alan always turns the aquarium light on before putting fish food into the tank. after a while, he notices that the fish swim to the top to look for the food as soon as he turns on the light. in this example, the light of the aquarium is the conditioned stimulus.

On the other hand, unconditioned stimulus is a response arising naturally. For example, you can get hungry by smelling your favorite type of food.