Koi are cold-water fish, but benefit from being kept in the 15-25 degrees C range. They don’t react well to long cold winter temperatures as their immune systems become inactive at temperatures below 10 degrees Celsius. The koi fish’s bright colors put it at a severe disadvantage against predators therefore a well-designed outdoor pond is very important when keeping koi fish. The pond should include areas too deep for herons to stand in, overhangs high enough above the water so that mammals can't reach in, and shade trees overhead to block the view of birds. It might be necessary to string nets or wires above the surface of the pond as to keep predators out. Also, include a pump and filtration system to keep the water clean and clear. Koi are an omnivorous and will eat a wide variety of foods, including peas, lettuce, and watermelon. Koi food is designed not only to be nutritionally balanced, but also to float so as to encourage the fish to come to the surface and feed. When they are eating, it is recommended to check koi for parasites and ulcers.