If you visit a castle in Japan, there is a moat. The moat can be filled with water or sometimes it is dried up. The moat is one of the defense system and not consider to attract the visitors = enemy.
On the other hand, if you visit a garden or a temple, there is a nice pond. These ponds are not for prevention of crime but to attract the visitors = patrons. Many ponds have koi, colored varieties of Amur carp, which make the pond much attractive.
Due to it is not a lake, there is no creek or brook connected with the pond. Therefore the water usually gets dirty and sometimes unwelcome animals can be living in the pond. These animals are invasive species; frogs, turtles or fish which were former pets. People bring to the ponds and release those animals when they gave up feeding them. Those animals can finally eliminate the native species living the ponds. Therefore ponds needed to be cleaned up. The major way to clean up is to remove the water, sewage sludge and pull out the animals. Then, leave the pond to dry up for a few days and fill with clean water again. Finally native species are released again to the pond(the invasive ones will be expelled).