Scary fish

I went to get my oil changed and a car wash. While waiting inside there was an aquarium with several small fish swimming around leisurely and one huge fish. The big fish would swim to the top corner of one side quickly go the the bottom corner of the opposite side and continue around in the square aquarium never stopping. I wonder if the small fish were annoyed with him. He was scary looking and seemed anxious since he could not find whatever he might have been searching for.