As a pup the BTR shark could be kept in 2.5 feet of water but only for a short period of time. When they reach maturity they will be much more comfortable and have a better chance in atleast 4-5 feet depths. These guys are true shallow water sharks in that there are many reports of adults cruising in as little as 18 inches, but that is not to say they can kept in such a shallow tank. There is also the shark's sense of security to consider. Even though sharks are top notch predators, they can also be preyed upon by larger predators and other sharks. If a captive shark doesn't feel secure in its environment then it simply won't live long. Too shallow of a tank will have the animal constantly 'spooked' and will most likely never settle in to its tank.
I would also like to add a word of caution, black tips like many open water sharks have problems at times with tank corners. This can be especially true in tanks that are a bit too small for them. Not to start another bashing session but there was another post here about a black tip that was in a glass tank in a pet store. Yes there seemed to be very good filtration and the like, but I can foresee problems for the shark as it grows. I have seen many cases of sharks at public aquariums that had damaged snouts due to their habitat. There is a litany of problems that can come from the damaged nose of a shark, bacterial infections, inability to find and carture food, and problems with navigation. Please do not think I am taking shots at his pride and joy, I am only pointing out a potential problem that I have seen personally in a professional setting.