Tea Leaves are good for this sort of flow visualisation - I have used them many times at work to try and understand water flow behaviour.

If you think about it its not surprising that constant uniform rotation does not agitate the developer - there is almost no relative movement inside the tank between the surface layer of the film and the chemical.

What agitation is supposed to achieve is to remove the depleted surface layer of developer with fresh chemicals.

Whether you use a flick of the "Twizzle Stick" or use inversion, the effect is the same, lots of relative movement between the film and chemical - allowing fresh chemical to replace the stale layer at the film surface.

On the subject of Paterson Tanks leaking, I have just invested in some new Paterson Tanks and have been pleasantly surprised how leak resistant they have been.

However, I still wear Nitrile Rubber Gloves - a friend after many years suddenly developed an allergy to Metol, which is a common component in many B&W Developers.

He nearly had to give up Photography because of it

Martin