There's a pet shop by me that does this sort of thing. They specialize in saltwater tanks. But the thing about this is you have to be REALLY good, and really know what you're doing. Most of the places they go are like, Doctors offices, or places of business that keep saltwater tanks for decoration, where no one wants to do any maintainance on them. Most places want the really colorful, energetic fish, not reptiles or amphibs or even freshwater animals.
I doubt there's enough demand in any one area to make any good money on it. But if you could convince a few places to let you setup and maintain a tank, I suppose you could make some side money on it.