There are two kinds of kreisels. The first type of kreisel (known as a pseudo-kreisel) has two vertical acrylic plates, parallel to each other, and as far apart as the plates are wide. Between these two plates is cemented an acrylic plate in a "U" shape that runs parallel to the two plates, and is as wide as the space between the two plates (obviously). On one side of the U (at the top), are is the return, that is divided into several pipes that run the width of the top of the "U", and point down. These returns push a sheet of water around the "U", keeping anything in the water column from settling. The second design, the standard kreisel, has two parallel acrylic plates close together. Between the plates, like in the pseudo kreisel, lies a "u" shaped piece of acrylic shaped into a wide semi-oval at the bottom, and it stops at the top of the tank. The returns at the top of the tank are identical to those on the pseud kreisel, except there is one on each side of the system, so as they fire water down the curved side, they meet in the middle, causing an upwelling, again keep the specimens settling, but also keeping them perpetually moving towards a more central area in the display, as standard kreisels are typically much longer and taller than pseudo-kreisels.
Chris
