If you build it to be natural, then don't be suprized if nature moves in. If you don't want them to move in, then build you a nice round or rectangular fountain with no fish. Of course, if keep out the bullfrogs, then the herons, owls, hawks, and eagles will delight in their meal of fresh fish. You can correct nature, but it don't stay corrected. If you don't want them in your pond, put them in your plate.
Happy ponding,
Scott