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. The apparatus comprises a first in-feed channel for a flow of living organisms, a second in-feed channel for a flow of fluid, a chamber containing a lever for regulating the flows from the first and the second in-feed channels through the chamber, an outlet, a computer, and a sensor. The sensor is positioned in the first in-feed channel for detecting the density of living organism in the flow, and the computer continuously and automatically regulates the ratio of flows from the first and the second in-feed channels through the chamber and towards the outlet by the lever in response to the density of living organisms in the flows determined by the sensor.
