Open spaces in the grow beds allow more sunlight in and algae, duckweed and watercress fill the voids
Watercress and duckweed growing in south side of the top grow bed
Lots of decomposing solids in the grow beds (soil, worm poop, dead plants and planktons)
The solids in the grow beds are removed in a UVI style system. In this system the solids breakdown in the beds releasing nutrients, minerals and possibly noxious gases (methane and hydrogen sulphide).
The trick is to balance the ratio of anaerobic decomposition with aerobic decomposition.
If to much anaerobic activity begins,
1. bacteria with convert nitrate back to nitrite which is toxic to fish
2. The conversion of nitrate to nitrite raises the pH
3. The combo of low oxygen, high nitrite levels and high pH leads to fish death.