The history of knowledge and media of the aquarium in the 19th century. With the invention of the aquarium around 1850, the underwater world became observable, researchable, and experienceable in entirely new ways. This represented a profound change not only for scientific research but also for the public representation of maritime worlds. Mareike Vennen traces the early history of the aquarium as a medium - from the first amateur scientific attempts with home aquariums in Great Britain to its spread across Europe and beyond. She follows both human and non-human actors: tinkerers, scientists, glass plates, pumps, dripping mail deliveries, water snails, and mud emerge as protagonists. Through a series of microhistorical case studies, it is shown which media - from field drawings to the first.