Mosellum is an information center beside the Moselle fish pass in Koblenz. Its exhibits explain river ecology, fish migration, and the engineering that helps aquatic life move past the barrage.
It is a focused stop rather than a major museum, but it adds an interesting environmental angle to a city usually visited for fortresses, river views, and the Deutsches Eck. It works best if you are walking or cycling along the Moselle side of town.