In order to advance a theoretical approach to settling velocity one must know density of the feed pellets. In this study a device to measure fish feed density was constructed. The device measures buoyancy force applied to the feed in the mercury and from the known density of mercury the volume of the feed pellet is calculated. The mass of the pellet is accurately measured with a precision scale and density is calculated from mass and volume data. The device is tested by measuring densities of some steel spheres with the device and Sartorious Density Measurement Kit. Both measurements agree within 0.1% which proves that the device is working well. The density of fish feed pellets is different for each pellet. As an example to the use of this device on fish pellets, we investigated density distribution of standard trout feed (6 mm) produced by Trouw company.