Common aquarium fish include the tetra, guppy, OPAH, and neontetra. Tetras are small freshwater fish that come in a variety of colors and are popular for their peaceful nature. Guppies are also small freshwater fish that come in many colors and are known for their hardiness. Opahs are large, colorful saltwater fish that are popular for their unique shape. Neontetras are small, brightly colored freshwater fish that are popular for their peaceful nature and ease of care.