The Friends of Belle Isle Aquarium history page (click through for some great photos & historic postcards) says:
It was August 18, 1904 in Detroit when architect Albert Kahn’s new aquarium would open to the public … The Belle Isle Aquarium, which opened adjacent with the new horticultural building on Belle Isle at a cost of $160,000, would quickly become, “one of the most popular attractions on the Island.”
…The interior of this aquarium were framed cypress tank-lined walls that were filled with fresh and salt water fish. The water contained in many of these tasks were brought direct from the ocean for the aquarium. Under the domed ceiling in the center of the building was a deep pool that was encircled by several small tanks. Later this pool would become the home to a large tank that would sit in the middle.
The most magnificent part of the interior was the grotto ceilings lined with shinny jade green titles, giving visitors a unique feeling of being underwater. Underneath this aquarium was a basement, that would be used by many as a speakeasy during Prohibition.