I have a friend (and customer) who has a outdoor reef pond of 2,500 liters constructed out of fiber glass and lined with bricks; its above ground .... I will post some pictures of this pond in the near future.
It was previously a Kio fish pond before he converted it ... it has a 2 hp chiller, a Aqua Medic T5000 Twin with bio tower, Calcium Reactor 5000, Denitrator 5000 and a 55w UVC from TMC running .... all these equipment (except for the chiller) were just recently added to allow him to maintain his corals better ... prior to that the pond had been running for the last 2 years.
Previously, the water was rather green as the pond receives direct sunlight for about 4 to 5 hours a day and it was using 2 macro skimmers that can only handle a load of 1,000 liters and still using the former kio bio filter ...
Anyway, it is a work in progress.