If you use chemical inputs (not so good...) you won't need a biofilter.
  1. Filter
  • At the bottom of the first tank, drill a hole with the hole saw of the diameter of your outlet pipe to the second tank.
  • Install the nozzle and hose on the tank
  • Spread a layer of coarse gravel on the bottom (1/4 of the filter volume)
  • Add a layer of clay beads of the same thickness
  • Add a slightly thinner layer of sand on top
  • Install the filter under the water pocket at the outlet of the gutter, and raise it above the biofilter to allow the water to flow by gravity.