Koi ( 鯉 , / ˈ k ɔɪ / , [koꜜi] ) or more specifically nishikigoi ( 錦鯉 , [niɕi̥kiꜜɡo.i] , literally " brocaded carp") , are ornamental varieties of domesticated common carp ( Cyprinus carpio ) that are kept for decorative purposes in outdoor koi ponds or water gardens .