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To create proper living conditions for sea mammals kept in closed systems, one has to make sure that the characteristics of breeding pools, such as their shape, dimensions, the facing of the walls and bottom, as well as the quality and motion of water, resemble as closely as possible the natural environment of the animals. An appropriate system of water exchange plays a very important role here. A complete exchange of water is time-consuming, expensive and troublesome, so it can be performed only periodically and should be supported by a supplementary continuous exchange. This operation improves water quality and can create a proper velocity field in breeding pools. The breeding pools investigated in the present study are located in a sealarium in Hel (Poland), which belongs to the Institute of Oceanography of the Gdansk University. Tracer measurements, carried out in these reservoirs made it possible to evaluate the intensity of continuous water exchange. It was found that this intensity was insufficient (as evidenced by large dead zones in the pools and short detention time), and therefore alterations to the existing system were proposed (i.e. a tangential position of the inlet and a centrally situated outlet). On the basis of a simplified model of circulative water flow, it was shown that the altered hydraulic system can considerably improve the situation.