My uncle did that once for cold water marine fish. He positioned a normal refrigerator under th tank and placed the external filter into the fridge so that the water entering the filter was cooled. However, the temp was not lowered as much as you might think, as the water did not spend much time in the refrigerator altohugh the temp of the fridge was set very low. You could posible place a large storage container of water in the fridge, then cut the tube OUT the filter in the middle and attach each half to opposite ends of the tub, so that water sits in the fridge for a while. You would probably need to put the tube emptying into the tub at the top of the tub, and the emptying tube at the bottom so you don't keep putting the same water round the circuit. On the other hand my idea might not work!!! never tried it!!!
Chris