The established freshwater bulls have been known since before I started working with sharks. Studies have shown MASSIVE amounts off urate (and the related ammonia) being released by the sharks body to keep it in check with a low/no salt environment - making FW under human care almost impossible. Back in the early 2000's we looked at bulls specifcially for a display with the idea of the idea of saving on a low salt/ no salt display for them. The water volume needed and the added filtrations required far out weighed the expense of running them in a marine display.