Okay, so people are carrying big fish in too small tanks then? If I ever get a CK it's gonna have a tank/pond that big or I'm not getting one at all.
Who says what's too small? Assuming the fish is healthy and the water is clean, is there really a problem? I love when people talk about a fish being "happy", especially when it's something like a clown knife that naturally would occupy an area the size of a swimming pool, if not a mile of river. Plus, plus when was the last time you saw a 36" clown? Only ones I've ever seen were at public aquariums.
Also, according to that formula, neon tetras should be happy in a tank the is 6" long and 3" wide...
If we really want the fish to have a perfect life, leave it in the river.