PURPOSE:To always maintain sea water in a water tank for transporting alive fishes in a sufficiently oxygen-absorbed state and further optimize the temperature of the sea water for the growth of the alive fishes. CONSTITUTION:A circuit for circulating sea water in water tanks 7a, 7b is separated into a cooling water passage A and an oxygen-supplying water passage B, and the cooling water passage A is further branched into a bypass passage P quipped with a water suction control valve V, thereby permitting an oxygen-supplying sea water feed rate optimum for the nitrification of ammonia for a biological filter medium 5.
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