When I was in Yr 11 at school I designed a billabong at my parents house as part of my horticulture assignment, then built a year or two after I started landscaping at about 19. It's still going along fine, the oldies have added a few more of their own plants since then, but it still copes fine with drought, flood but most work is done by plants, fish, frogs and moving water.
Rainwater pipes from a nearby pergola fill the streams and pond, overflow goes into a charge system into a 13,000Lt tank buried underground. Comes alive at night particularly during summer. Never really had issues with mould, mozzies as long as the area is healthy, fish are there, and water kept moving by the waterfall. A few fish lost their battle with a few Heron's and Ibis's but there's plenty of hidey holes now!
Full writeup and pics here-
http://www.plannedlandscape.com.au/sust ... ecosystems