In a bad economy, you'll be hard pressed to sell these to businesses that are scraping for pennies - something that requires a lot of time and money with no ROI. In restaurants, they can be another thing, that if not perfectly maintained, make your restaurant look gross.
I think "fish people" can imagine how great it would be if there were fish tanks everywhere - but people who are not fish people do not get it. And it's hard to make a non-fish person a fish person.
Not to be overly negative, but to put a different perspective on it. I wouldn't quit your day job. But I assume there is probably someone in your town typing into Google "[city] [state] custom aquariums" or something at least on a semi regular basis - but is the demand enough to make it a "real" job? You'll only find that out with time.
In my experience, every where I have gone with a fish tank, usually one of the employees really likes fish and takes care of it after hours.