To attain excellent cooling effect, realize the cost reduction and energy saving, and exhibit high resistance to corrosion in the cooling system that is attached to the water tanks for tropical fishes and cools the water in the tanks so that its temperature may not exceed the suitable one for tropical fishes.

This cooling system for tropical fish tanks comprises a casing 15 that is set in the water tank, a filler 16 that is contained in the casing 15 to increase the exposure effect between water W and air A, an air-blowing fan 17 that is set over the filler 16 and blows the cooling air A into the casing 15, a cooling water supply system 19 that feeds the water W in the tank 2 over the filler 16, and a sprinkler 18 that is rotatably arranged between the filler 16 and the fan 17, and is rotated with the air from the fan 17, while the water is fed up to the position over the filler 16 and sprinkled uniformly over the filler 16.