Since aquariums specifically are what you are looking for, I'd suggest primarily looking for major cities lacking an aquarium that are alongside major bodies of water. It seems most of the major aquariums out there are situated close to a major waterway, perhaps logically so due to the large quantities of water needed to run an aquarium. For US aquariums, that is sometimes the Atlantic Ocean (e.g., New England Aquarium in Boston, Mystic Aquarium in Connecticut, New York Aquarium, National Aquarium in Baltimore, etc.), sometimes the Pacific Ocean (e.g., Oregon Coast Aquarium, Seattle Aquarium, Aquarium of the Pacific, Monterey Bay Aquarium, etc.), sometimes a Great Lake (e.g., Shedd Aquarium in Chicago), major river (e.g., Mississippi River Aquarium in Iowa), or sometimes the Gulf of Mexico (e.g., Florida Aquarium, Audubon Aquarium of the Americas). Of course, there can be great aquariums further from waterways- Georgia Aquarium being the most notable one, but these seem to be the exceptions, not the rule. So while if you have a good idea related to another city, go for it, but if you need to limit your options I'd suggest a location near a major body of water as being one litmus test you could use.
