This isn't a bad way to start out. I don't know what your housing accomodations are, but if you had the room you could build one of those steel building for cheap these days.. especially if you think of 12 months rent, and on top of that most places want a 3-5 year lease agreement. With your own building you don't have that, plus it would bring value to your home, also with a new steel building it would be easy to run the plumbing and when they poured your cement slab just have them pour a few extra walls and bingo you have yourself a new monster fish tank... it could easily be a few thousand gallons. Stock it wisely with fish that would bring customers in and you now have a great attraction. All while being on the edge of your property and if you were get out of the hobby and close up shop...
well now you have an awesome garage with a awesome tank. Any mans dream...
Good luck...