As you tilt the tank, the freshwater and saltwater flow quietly right and left respectively. Seconds later, the boundary surface minutely forms a wave, which grows quickly to a beautiful arabesque design, then within the blink of an eye, crumbles. All within the space of roughly one second.
Mathematically speaking, a solution for the equation in which the freshwater and salt water flow to either side without mixing is possible. However,
Nature doesn't like it.
For some reason, Nature selects ever-increasingly complex currents, and finally ends up with a turbulent current in which two kinds of fluid mix violently.