This paper proposes a new approach to construct a real-time monitoring system of aquaculture fish based on an underwater wireless sensing technique of low-frequency (LF) RFID. Behavior of tagged fish in aquaculture tanks can then be identified dynamically. With the culture and breeding traceability data, efficiency and precision of broodfish management or running an aquatic genetic resource bank can be improved. The testing and proofing facility is a small-size FRP fish tank with 1.5 m diameter and 1 m depth inside. HFSS, a visualization simulation software, was used to numerically analyze the H-field and E-field strength distributions in near-field space of square LF loop antenna in the first place. Result showed character of magnetic field of 30 cm size-length antenna was better than other sizes and less impact of H-field strength in fresh water. Real object testing result showed perpendicular angle, between x-y plane of antenna and center axis of RFID-tag, would reach the largest interactive energy and readable coverage. Critical trigger voltage of emulative coil of RFID-tag was measured to be 1115 mv against the antenna while the calculated H-field strength was 16.6 A/m. Underwater RFID antenna system in the fish tank was constructed by combinative ways of 4-quadrant and antenna arrays (4 or 8 arrays used, 2 or 4 antennas for each array). A single LF loop antenna was measured its RFID-tag related maximum readable ranges onsite in water and in great detail. Thus, the whole readable area crossing x-z plane of each kind of antenna arrays, totally 16 or 24 antennas, could then be calculated to be about 55% or 85% of the horizontal circle plane of the tank, respectively.