Many designs for biofilters have three stages in one container: (1.)settlement areas, which are under the (2). mechanical filtration:a layer of furnace filter or batting, burried under  the (3).Bio filtration medium,  such as lava rock.

     Well I tried this and it's not too bad.  When you need to clean that burried layer of batting or furnace filter you will have to remove all that lava rock. Lava rock clogs and it is heavy, netting does not have much surface area and screen clogs easily.

     I do many lily repottings and generally mess around too much in the pond- as a result  I need to clean out my mechanical filtration media frequently , so I seperated each stage of the filtration  process , facilitaiting access  to the areas which needed attention.

     * Seperate each stage of the filtration process  and

       *find  an effictive bio  medium for biofiltration.

       * Vegetation is  a fourth stage of filtration

  Bio filters really do not filter but convert  the chemistry of the water to  conditions that are safer for fish. But they do create  a good sorce of nutrients for plants, including algae.  THerefore  plants are necessary. Early vigourous growth of cattail and irs consume nutrients which would otherwise encourage algae growth. The cattails and iris are  not set in soil so their roots must draw nutrients from the water.  These are the most helpful plants during the period which is too cool for most other plants.

1. bottom right is the settlement chamber ,
2. bottm left is the  mechanicahl filtration where water flow down through  batting or foam and is filtered  before if reaches the pump at the bottom of the tank,. then it is  pumped to the
3. bio filter  in the  robber made tank  filled with  plastic ribbon or spring flo