For me, aquariums have come to symbolize more than just a hobby. From engineering, chemistry and biology to design and user experience/learning, aquariums represent a mixture of disciplines that need to be combined to be successful.
It isn't simply a way to spend mindless hours watching pretty fish swim by; it's construction, research, monitoring, improving and being creative. And, it's instructional; aquariums teach patience, perserverence and incremental and iterative development.
In other words, it's rewarding, calming, problem solving, and fun.