Large, deep koi ponds rarely use gravel, since the gravel is hard to clean when it is 6 to 10 feet down ... Ponds with few fish can get by with smaller filters, but fish grow and filters don't, ...

The ponds use permeable materials like gravel to filter water that eventually becomes drinking water. By Judy Peterson | Bay Area News Group UPDATED: March 14, 2017 at 2:14 PM PDT ...