Well, I combined my photography skills with some other talents ( underwater) and that got my start in a good business. So, I would really differ with your opinion on that. What I did learn is that underwater photography was indeed limited, so I went commercial, then Broadcast video. But my start was in film 16mm and later super 8 and also 35mm still. There is lots of demand for photographers but you have to do it professionally and the best way to get started is to work as a free lance for another photographer as his assistant to learn the ropes. If you are good and own your own equipment, you can soon be in the business making a decent living on something you love to do.
Aquariums- I did a TV show a couple of years ago on this and the owner of the store got started as a service maintaining aquariums for busy people, next was a store, and today, he has a thriving business. Yes, it's not big with lots of competition, but if you establish yourself in a community as the only one, you'd get all the business. When I lived in Middletown NY, there was only one store in the town for miles around and they were always busy. Then again that was a few years ago.
Maybe you should examine two roads- One is to make what you have fun doing as a business, or the other way- get something like a franchise and put up with employee problems, retail issues etc. Since I had my fill of managing lots of people I didn't want that hassle again. That's what drove me down the first road.