A flexible tubing for fine bubble aeration is provided with an air passageway defined in part by an upper portion and a lower portion. The tubing can be made of a uniform weighted material with more material in the lower portion than in the upper portion. This makes the tubing self-orienting, in that it will tend to orient itself with micro-slits along the upper portion facing upward and the lower portion facing downward when submerged in a body of water. An automated, one-stage production line converts raw tubing material to a finished tubing product without the need for separate processing. A method of coiling the tubing places the micro-slits approximately 90° away from the surface of a spool hub, thereby avoiding a longitudinal arch in the tubing and ultimately preventing roll-over and improper slit orientation after installation in a water body.