A lot to say, I'm afraid.
Here's my objection... Though I think in principle this is a very good idea, many ppl will be quite stunned by the cost of having and running an aquarium. The ppl in these forums are performing a labor of LOVE by keeping fish. They don't mind being wildly rash with their money and time. Their fish survive as a result. When a business has an aquarium, this usually indicates that one of its employees has this same passion, and will spend extra hours working. If YOU have the passion, and not the business owner, things could go wrong. You might end up working a lot of extra time without pay, because you believe it is right for the fish, and they aren't willing to pay for extra work.
The bright side... It might work. But I'd only advise pitching this to businesses that can afford it, and have a fair amount of space. Some examples would include: a new-age restaurant, a nice hair salon, a museum, an insurance brokerage, a legal practice. Tanks are especially good for businesses which seek to draw customers to take a closer look at their store. Also, nursing homes, schools, as you mentioned. Places like this might go wild for the idea. I'm sure that a college town will have at least a couple of places like this.
When doing your pitch, bring along really really good pictures of your tanks, or other tanks you wish to emulate. Pictures really sell an idea. Check out the store beforehand, and suggest a possible location for the tank, showing how its presence might entice customers to enter the store. A tank that has both sides free--one to the window--and the other inside, would be perfect for kids, and would give parents a chance to relax. Also, bring along a budget, and don't play down the cost. Aquarium-keeping is notorious for hidden costs (medication, aquarium salt, new decorations, replacement fish, etc.), so give them an upfront estimate, and a monthly maintenance fee.
Why do I think I'm qualified to tell you this?? Well, I'm in the process of setting up a Lake Malawi cichlid tank for one of the departments at Columbia University. I proposed the project, and was brutally honest about the cost of getting ready and maintaining a 100 gallon tank (that they had already bought), and gave them a realistic rundown. They did accept my proposal, and the tank is going on line some time in the next few months. Your employers will appreciate your up-front honesty.
So, good luck, and get a great pitch ready!