There may be N number of reasons to keep an aquarium, but I believe the most important one is to manage a time out to truly think of the world we live in, but not to think of the world we made, simply!
We all know which one is a pollutant, we all blame each other, we all curse natural calamities, we all curse growing unawareness, but still we practice the causal habits!
I don't claim that aquahobby can directly help overcoming that, on the contrary some activists can accuse the hobby too!
You come home after myriad issues of our financially driven society where still money matters much more than anything else! Who else thinks of to get up early in midnight to wait for a comet which only passes by each 75 years, who among us is very anxious about the tiger disappearing in sunderbans! We, the common people, think, that we need to awake up in the morning, need to go market to buy a fish for food, then will eat it and start heading for the job place! That's a heck of a routine to follow up whole day, tomorrow and the day after tomorrow!
This hobby, can take away you from this man made 'virtual' world which we call reality and day by day as you grow up as a true hobbyist you learn the lessons of nature in manifold manners! Still, you are not contributing, but now you are a changed person!
Next time when you see an old banyan tree in your office campus, I doubt you can overlook that and just pass by! Because, now you will be amused to see that tree, its aged trunks, labyrinth of complex root system, vigor of its leaves! You know it better how much pain you took to grow that in your glassbox!!! Now, whenever I see a small epiphyte is happily growing in creeks and crevices of bricks and mortar, I think of my anubias which struggle to throw in a new leaf in a whole month!!! I envy nature -- then I realize in the next moment how infinitesimal we are in front of nature!
I believe, I am able to justify my points to keep aquaria at home!