Ferrofluids are fundamentally unstable. They're colloidal suspensions that rely on surfactants to keep the particles apart, and as the surfactants degrade, the particles start clumping together and falling out of suspension.
As a practical demonstration of this, there's an ancient ferrofluid exhibit (80's?) at a local science museum. While the fluid itself kinda-sorta "works" in that you get the pretty spiky shapes when zapped with an electromagnet, the edges and bottom of the glass case are thickly coated in viscous black gunk.
https://en.wikipedia.org/wiki/Ferrofluid#Description
As a practical demonstration of this, there's an ancient ferrofluid exhibit (80's?) at a local science museum. While the fluid itself kinda-sorta "works" in that you get the pretty spiky shapes when zapped with an electromagnet, the edges and bottom of the glass case are thickly coated in viscous black gunk.