@streetsmart
A pool fence is planned. It will be a removable job for when kids are visiting.
Something like this:
http://protectachild.com/
There will also be a grab bar put into the pond to facilitate exit if someone falls in. It will be one extra thing the koi might injure themselves on, but is a necessary risk as the pond is a little too deep to climb out of unless one is quite fit.
The ledge around the base of the pond was also put there to provide a foothold for someone without the physical strength to climb out.
@nutx
Under the BD is concrete stabilized soil. We had to dig a trench in that as the 6" sweep elbow we used has a 2 ft span. It and the rest of the base was sealed in concrete. The wire was then built on top for the reinforced base.
As for the issue with algae, a UV light inside the mechanical filter toasts the drum 24x7. That should take care of the bio film growth on the drum. I'll also be running the system at just 1/2 to 3/4 the 15000gph max capacity to allow more buffer.