PROBLEM TO BE SOLVED: To provide a water purification device for a water tank for cultivating fish and shellfish or breeding aquarium fish, capable of reducing frequency of water exchange from the outside by suppressing a rise of a concentration of nitrous acid.SOLUTION: A water purification device (1) comprises a sand filtration device (5), a biological purification treatment device (6), and an electrolytic treatment device (7). The sand filtration device (5) includes a filtration material (10) of filter sand, and force-feeds and filters breeding water of a water tank (2). If suspended matter is accumulated, the water purification device (1) back-washes the filtration material (10). The biological purification treatment device (6) includes active stones (28) comprising granite porphyry. Ammonia and nitrous acid in the breeding water are treated by nitrifying bacteria adhering to the active stones (28) and are changed into harmless nitrate. Air or ozone is supplied to the breeding water supplied to the biological purification treatment device (6) as the need arises. The electrolytic treatment device (7) electrolyzes the breeding water, and appropriately discharges acid electrolyzed water to the outside. Thereby, reduction of pH can be suppressed as a whole.
