Ravnis mentioned greenwater aquaculture on another thread and thought that it was worth discussing in its own thread.
You can read an article on greenwater aquaculture by a UVI researcher John Martin......here.I have trialed growing fish in a greenwater culture with the algea growing to be the means of nitrification. It has worked well, but growbeds will not function with the thick solid mass that the agae forms.
I have set up a series of tubs that let the algae settle out of the water and pump it back into the tank. While I can't directly hook up grow beds to this system, I have found two good uses so far. One is that my ducks go wild for the algae and the red midge fly larvae(blood worms?) that grow in it. The other is I throw it on my small trial wicking worm bed and the growth is good. I have a watermelon plant that has vines that are ~10ft long and has produced 3 medium sized water melons. The worms either eat the algae or the bacteria growing on the algae. The only main issue I have had has been aphids, ants, and grasshoppers.
The ph of the water runs around 8 and will raise back to that in a matter of hours even when acid is added to bring it down.
Vivienne Hallman, a Brisbane scientist relied on greenwater nitrification when, several years ago, she raised silver perch in tanks that had no bio-filtration. You can read about her efforts.......here. Her work is important to local micro-farmers because it confirms that greenwater culture is viable (albeit at much lower stocking densities) for some Australian species.
I think that Jim Fah's AutoPot Aquaponics system also uses algae for nitrification.
Greenwater culture is an appropriate technology idea that certainly deserves a closer look.