Film developing tanks are containers used to process photographic film in a dark environment. They allow photographers to develop their film at home, providing control over the developing process. These tanks typically come with a light-tight lid and a reel that holds the film securely while it is submerged in chemical solutions. The process involves loading the film onto the reel in complete darkness, then placing it inside the tank. After sealing the tank, photographers can add developer, stop bath, and fixer solutions through a funnel. This method is essential for those who prefer traditional analog photography over digital methods.