## 5 Conclusions  
In this work, the motion (flow pattern) of solid feed pellets in a fish tank was simulated (using ANSYS Fluent software), in order to discern pellet suitability for aquaculture systems. First, we designed the experiments to determine the effective density of feed pellets in water (based on the settling of pellets in a water column and determining the settling velocity). Having determined the properties of the pellets, 3D CFD simulations of fluid flow and the distribution of feed pellets in a fish tank were performed. The mean residence times which might be one of the important parameters in the design of fish tanks were evaluated for various operating and geometrical parameters, namely pellet characteristic size (diameter), volumetric flow rate, density, and the location of the pellet injection. Knowing that the quantitative results depend on both (i) the specific design and operating conditionsof the fish tank and (ii) feed pellets properties, we present some of them. The mean residence time of pellets injected at the surface of the fish tank can differ by up to 75% depending on the injection position. The shortest residence time is when the position is located at the center of the liquid surface (17 s); the largest is near the edge of the tank (75 s): The maximum difference between the studied two types of pellets is 25%, and it increases with position closer to the center of the tank. The maximum difference for positions along the perimeter at 3/4 tank radius is 8%; the most extended residence times are observed at the opposite side of the water inlet.  
As far as we know, this is the first study dealing with both the identification of properties of the pellets and correct settings of the model in CFD solver as well. Such a CFD model might be beneficial for researchers and aquaculturists who are concerned in fish farming and looking for optimal properties of the feed pellets, as well as optimal design and operating parameters of fish tanks.  
Concerning our future goals in this research field, we want to decrease the discrepancy between CFD simulations and a real system. One thing is reflecting the change of pellet properties with time, and the other one, quite challenging, is the effect of fish on the flow field and possibly the particle (pellet) distribution in the tank. This might be neglected for small fish and a small number of fish, but the larger fish amount can substantially impact the system's hydrodynamics.  
Conceptualization, S.P., K.P., M.S., V.S., and P.C.; methodology, S.P., K.P., M.S., and V.S.; software, K.P. and M.S.; formal analysis, S.P. and K.P.; resources, S.P., K.P., and V.S.; writing--original draft preparation, S.P., K.P., M.S., and V.S.; writing--review and editing, S.P., K.P., M.S. and P.C.; visualization, K.P. and M.S.; supervision, S.P.; funding acquisition, S.P. and P.C. All authors have read and agreed to the published version of the manuscript.  
The study was financially supported by the Ministry of Education, Youth and Sports of the Czech Republic, project CENAKVA (LM2018099), and NAZY project (QK1810296), and OP RDE grant CZ.02.1.01/0.0/0.0/16_019/0000753 \"Research center for low-carbon energy technologies\", and the European Union's Horizon 2020 research and innovation program under grant agreement No. 652831 (AQUAEXCEL2020).  
The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.