The wave in the clip at right reflects between the walls of the tank. The water sloshes back and forth. With no friction, in the ideal world of a flash movie, this goes on forever.
This wall-to-wall bouncing is called "seiching". It occurs for both surface waves and internal waves. For a given wave speed, and a given distance between the walls, there is a particular period of these seiche waves called the "resonant period" of the system.
The period is given by ...
... where L is the distance from wall to wall, and v is the speed of the waves, whether they be surface waves or internal waves.
Suggestions for further work
1 Use the effect to measure the velocity of deep-water surface gravity waves as a function of wavelength.
2 Use the effect to measure the velocity of deep-water internal gravity waves as a function of wavelength.
A seiche calculator is/was at ...
http://www.coastal.udel.edu/faculty/rad/seiche.html