Watery Milieux. Ecological Perspectives in Marine Biology and Aquarium Studies around 1900. The paper focuses on the research spaces of marine biology around the turn of the 20th century and argues that the aquarium played a decisive role in shaping the notions of environment and ambience, ‘milieu’ and ‘Umwelt’. By “virtually pushing” biologists to pay attention to the relations between organisms and their surroundings, the aquarium can be understood as a material inspiration for ecological theory formation and as a structural point of reference for thinking about the properties and range of environments. By focusing both on the academic discourse and on popular ‘Aquarienkunde’ the paper thus describes how marine biological studies became an important arena for the formation of modern ecology.