I tried it at a time when there wasn't much competition and even then it was impossible to break even. And I had it down to a near industrial level, so formalized that my mom could do it. The issue are energy cost (light, heat, pumps), water and of course space. It's no coincident that many of the more efficient hatcheries are in locations where they can get at least some of those factors for free.
Then there is the issue of selling off stock. Every fish you can't sell on time cost you money and is taking up space you can't use for the next batch. So you need to have a reliable customer base that take the fish you breed off your hands - not an easy task...
And finally, when you get large enough you may need to hire staff.
All of this doesn't even account for food cultures in case you want to breed something other than clownfish (clownfish can be bred with brine shrimp, frozen and dry food alone).