i think its more like one of those pet water bowles with jug that you turn upside down that provides water as the bowl empties. as the water level lowers it allows air into the jug until the water level rises over the opeing closing the path for air to flow in.
the water doesnt come out because of the lack of air creating a vacum.the tank would have to be sealed , filtration would not be a problem because it only pumps water in and out of the tank , not air, just my theory