Ten colorful koi glide around the Zoo's Terrace Lagoon in an 18,500-gallon pond, along with tiny mosquito fish and a bluegill or two. The pond has been around for decades, initially serving as a bird pool. In the 1990s, a filtration system was added to support the koi. Koi are ornamental varieties of domesticated common carp that originated in Japan. They are kept for decorative purposes in outdoor ponds with very clean, oxygenated water and plenty of shade (the fish are susceptible to sunburn!). There are over 100 varieties of koi, which can be red, white, black, yellow, brown, gray, and green, and any combinations thereof. Koi resemble oversized goldfish and, in fact, koi and goldfish are both types of carp.