An expression expressing the ideal state of weirdness. Many of us think that as long as we have avoided the drum of the masses we are satisfactorily unique, but when mixing metaphors, you go all the way or you keep your mouth shut.
Are you marching to a harmonica? Not good enough. A harpsichord? Better. But no. Those of us out of the know go with the potted piscis.
It should be noted that these days marching to a kettle of fish (even a different one) is getting quite common, leading many other marching styles. There are bumper stickers declaring that the driver is 'Marching to a Different Accordion', homepages advocating 'Marching to a Different Waltz', and the ever-descriptive 'Marching To A Different Paradigm'. It is debatable, however, whether these are weirder than the aforementioned kettle, or merely more original.