^ bingo, when planning out a tank you need to think on full size of the creature. I want a harlequin tusk fish, but cause I want one doesn't not mean I can go out an buy it. I can not property care for it so why would I go take this fish that was collected of a reef somewhere and give it a less then a proper life?
Also back on the subject of a shark, all sharks have a 6th sense. This is a electromagnetic force put off from special sensors on there snout. This allows them to pick up disturbances in the water. Because of the sharks travel in circle like patterns in, and around reefs, and when you put a shark into a rectangular tank they will bang their snout against the corners, rock work, everything really as its disrupting their normal swimming patterns. This hurts the shark, much like a human severing a nerve bundle in thier lower spine, both can lead to death as they cant hunt with out this
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36 gallon fresh water