My parents live in a rural area just out of Auckland. A few years back my father decided to dig a goldfish pond in his front yard. The front yard slopes down to a swamp/stream. There are various species of fish life, including wild goldfish of which he has caught a few and put in with the regulars. They are very dark silver/black, same size and shape, hang out with the other goldfish and, according to my farther, gradually change colour back to the gold we all know.
He pumps water from this stream to water the garden and irrigate the lawn when it's to dry. Through some clever plumbing this pump also feeds the pond. Water changes are done from the creek via a 44gallon plastic drum filter under the deck and the pond water can be circulated through the filter also. The filter feeds a small pool at the top with a waterfall down to the first pond, which overflows down to a second pond.
The water was originally drawn from the second pond for circulation, but he found that by the time the water had filled the tubing, the filter, top pool, waterfall and so on, the level would drop to about half.
Out with the spade again to dig an under ground holding tank from where the water can be drawn without affecting the water level of the second pond. The second pond overflows into this tank through a meshed grate which catches the leaves and rubbish from the trees around it. When I say underground, it has a large wooden lid, covered with bark and is planted. It does have a small inspection cover which he removed for the first time since he can't remember when, only to see a goldfish swim by.
He’s currently investigating ways of catching the poor bugger.