The term 'Popular Aquarium Denizen' can be summarized as the Betta fish, which is a five-letter word. Betta fish are a popular choice for aquariums due to their vibrant colors and interesting behavior. They are native to Southeast Asia and come in a variety of colors and patterns. Betta fish are also known for their ability to live in small spaces and their hardiness, making them an ideal choice for novice aquarists.