Sealed box. The Fluval A203 UVC In-Line Clarifier External Filter is a powerful filter designed for aquariums up to 100 gallons. It is an external filter that can be easily installed and maintained. The UVC clarifier helps to eliminate green water and other harmful bacteria, ensuring that your aquarium stays clean and healthy. The Fluval brand is known for its high-quality aquarium products, and this filter is no exception.