i dont agree with the width being shorter than the fishes length but imo if the width is 1" more than the length i think its ok, but the tanks length should be 4x the fishes length
Depends on the fish. A 16" eel thats as thick as a dime is perfectly fine turning around 12" tank. Should a 13" midas, oscar, arowana, ect turn around constantly in a 12" tank? No. Certain need a tank wider than the full lenght of the fish.
the fish arent gonna turn around in a straight line i dont think. they would swim either down or up and turn around at a diagonal angle if you can picture what im thinking. but i still dont think its optimal..