The bright colors of koi put them at a severe disadvantage against predators; a white-skinned "Kohaku" is a visual dinner bell against the dark green of a pond. Herons, kingfishers, otters, raccoons, mink, cats, foxes, badgers, and hedgehogs are all capable of emptying a pond of its fish. A well-designed outdoor pond has areas too deep for herons to stand, overhangs high enough above the water that mammals cannot reach in, and shade trees overhead to block the view of aerial passers-by. It may prove necessary to string nets or wires above the surface. A pond usually includes a pump and filtration system to keep the water clear.