## 1 Introduction  
Modern aquaculture starts to be more reliable upon using recirculating aquaculture systems (RAS) as an effective way to produce large quantities of fish using small footprints for building [1]. The rule of thumb in such intensive systems is to remove solid particles or uneaten food as soon as possible. In some system designs, in-tank solid particle separation by devices as a dual drain and particle traps,or next to tank devices based on sedimentation are used [2; 3]. Such devices involve vortex separators, radial flow vortex or lamellar settlers as a solution for particle removal including uneaten feed [4]. For such operations, the quantification of feed sedimentation rate or modeling of in-tank (device) feed movement is of high importance to optimize rearing and operating protocols. Some of the recent activities of large aquafeed manufacturers are to develop feeds specially designed for use in RAS. Manipulation with temperature and pressure during the extrusion process allows influencing the bulk weight (effective density) of feed produced.  
In some cases, ingredients such as guar gum or cork granules are added into the mixture to produce more compact or floating pellets [5; 6]. This requires checking the physical properties of produced feed including settling velocity. On the other hand, some fish species prefer slow sinking or semi-floating pellets as they are not aggressive feeders (pike perch). Therefore, manipulating feed settling velocity plays an essential role in fish production [7; 8]. For all mentioned applications, the modeling of a particular feed-in system of given hydraulics is of high importance.  
Here, we see the knowledge gap between (i) the general studies of the hydrodynamic flow field in aquaculture tanks [3; 4; 9; 10], and, (ii) outstanding works on the characteristics of floating particles, e.g., feed pellets, faecal solids, [7; 8; 11]. The third research area, relevant for our study, concerns the investigation of two-phase flow systems, i.e., bulk liquid continuum and solid particles [12; 13]. Let us emphasize that, while most CFD studies on aquaculture systems are focused either on specific problems related to the equipment design [3], or problems related to numerical issues and method validation [14], in this communication, the authors aim to present the original methodology, showing how the CFD study of the motion of different solid feed pellets is used in order to evaluate their suitability for aquaculture systems with specific fish. This holistic approach represents, at the same time, a scientific novelty and a way of using advanced computational methods. (Let us remark that the idea of integrating sub-models from various disciplines as computational tools (termed \"virtual laboratories\") for designing and checking experiments in aquaculture systems (before the real implementation in the corresponding research or production facility) is being exploited in the frame of the European Union's Horizon 2020 research and innovation program AQUAEXCEL 2020.)  
The paper is organized as follows: the theoretical background and some previous results for both experimental and numerical part of this study are presented in Section 2. CFD setup and results from the simulation of two-phase flow within the fish tank (see Figure 1) are presented in Section 3 and discussed in Section 4. Finally, conclusions are drawn in Section 5.  
Figure 1: **(left) Illustration picture of the fish tank operated at the Research Institute of Fish Culture and Hydrobiology, University of South Bohemia in Céské Budéjovice. (right) Tank geometry generated in ANSYS CFD software package.**