dont put a hole in the liner
build it with veggie filter over the pond
build a veggie filter in the pond (for winter)
everything spills over into the pond, not out of the pond
if there are fish, build in a way to net or otherwise protect the pond
in freezing areas build it deeper and a way to put plastic over the pond to keep the
water open.
once the right number of koi are in the pond, dont add any more, no new fish
Ingrid