A.K.A. a Pythagorean cup.

This was printed as a test object to demonstrate how bell siphons work for aquaponics systems. It will need to be water and air tight, so print it with 4 shells at .5 single wall thickness. If your printer has a different wall thickness, download the .scad and adjust accordingly.

Print one tray and one bell, then you might want to do some finishing on the parts to make sure they're watertight. Then the bell sits in the hex shaped cutout in the tray, with the holes down by the floor of the tray.

Begin to fill it with water... it will fill normally, until the water level gets to the top of the inner pipe, at which point all of the water will run out through the hole in the bottom of the tray.

This effect is useful for aquaponic and hydroponic systems, where a passive mechanism is needed to repeatedly flood and drain grow tables.