The invention comprises a method, a device and a fish-farm for controlling the concentration of living organisms in fluid in order to facilitate the handling of the organisms such as grading and counting. Said apparatus comprises a first in-feed channel for a flow of living organism in a fluid (1), a second in-feed channel for a flow of fluid (2), a chamber (4) containing a lever (11) for regulating the flow from the first and the second in-feed channel through the chamber, an outlet (15), and a computer, wherein a sensor (9) is positioned in the first in-feed channel for detecting the density of living organism in the flow, and wherein the computer continuously and automatically regulates the ratio of flow from the first and the second in-feed channel through the chamber and towards the outlet by the lever in response to the density of living organism in the flow determined by the sensor.
