A device that is used to draw water out of an aquarium at a controlled rate for delivery to a trickle filter, which is a type of awuarium filter. Visualize this:
Two smallish plastic boxes, one inside and one outside the tank, right next to each other, up at the top. There is a siphon tube connecting the two that must be primed with water. In the outside box, a hole is cut in the bottom to which is connected a hose that leads to the trickle filter. At the bottom of the trickle filter is a water pump that returns filtered water to the aquarium, so that the water level in the aquarium eventually rises above the internal box, spills over into it, and is siphoned out again. They retail for about 50 bucks.