The carp in the channels are living in spring water. Shimabara sits on ground that produces a great deal of it, and the water running through the channels in the town — the water the carp are kept in — has come up from below rather than been diverted from a river. Around Shinmachi there is a stone-paved street of samurai residences, and on the Shirachi side stand townhouses fitted with mizuya.

A mizuya is a room built to bring spring water into the house and use it in place, with the supply divided so that the water for drinking and the water for washing are kept separate from one another. The abundance is the point. Where every household can have its own water, there is no need to share a well, and the whole social apparatus that grows up around a shared well — the rota, the disputes, the rules — never had to be invented here. Shimabara is connected to Kumamoto by the Shimabara railway and by ferry across the bay, and the town has its own hospitals and shops, so daily life does not require leaving it.

A town with this much spring water does not need rules about using it. Nobody is competing over how much can be drawn, so there is little friction between upstream and downstream households, and there is enough surplus to keep ornamental fish in the public channels. That last detail is usually read by visitors as decoration, and photographed as such. It is more accurately read as a measurement: carp living in a street channel means the water there is both clean enough and plentiful enough to be spared for them.