Open any Good Pond filter and you will find these items, A case, some sort of screen. Bio-media(anything to hold bacteria), pipes. The following are examples of some of the better filters on the market.
Some have plants, some have UV lights also.
Filters that have bottem in-pipes that make it easier to back flush and drain off solids. This also helps keep the filter from becoming "An-aerobic". This first filter is good for about 200 gal of water, because it has plants well established in the top. Without the plants figure about 50 gal. if this containor holds around 45 gal. The plants have a tendancy to clog the out flow pipe and this filter can overflow out over the top.
On this next filter the Manufacture claims "500 gal" the filter is just larger than 1 cubic foot. Notice bottem flow design, plastic biomedia(bio-balls? Plastic Army men?Hair currlers?), dimple foam(sort of like the stuff used on hosptal beds) and Gravel. The UV light will set you back a few hundred dollars.
This next one has a "top flow" design, notice it says "Water Level Rises as foam becomes blocked" I do not see a return overflow pipe, when this filter gets "clogged" and what about the "Aerobic Activity" when the water stops bringing fresh oxygen Can we Say "An-Aerobic"
This design is known as "a Time Bomb" It will fail sometime , usually when you are not home. Does this filter work... YES! and it does it very well, as long as you keep the foam clear and not clogged.
This one is fun and grows many different kinds of plants very well. The plants make use of the Nitrogen converted by the filter below this works well for a small pond of 500 gal. Notice: Foam, Plastic Bio-Media, Gravel, Plants.
Your plant roots will extend down into the foam, quit possibly follow and clog the vent. as you can see the warning "Keep Clear" This one has a tendancy to overflow out the top, by floating the root clogged planter up
This next design has me thinking "How do they keep the clay from sliding down to the bottem of the pond?" The Pebble beach is a nice "Biofilter area" if they had added a pump to circulate the water, then bare root plant some marginal plants in the pebbles and you have a low maintainance pond !
The Bog Bio-filtration Method: The pump, moves the water off the bottem of the pond onto the pebble beach, The "Bio-media" (pea-gravel) holds the bacterial that converts fish poop to Useable Nitrogen that the Bare rooted Marginal plants suck up and grow more plants. The water is then allowed to flow back into the pond.. This system cannot clog and drain your pond. Figure 1/3 the size of your pond surface for the size of the bog. The bog should be 6 to 8 inches deep. If you want to put Bio-balls, or plastic curlers, or army men in with the gravel you can. But I would leave the sponges out, mix in some pumus rock instead. The bog should be shoveled out and the pea-gravel washed about once every 2 to 3 years. The once in the spring and once in the fall, remove and brown, dead plant material. Your pump should be in a basket covered with rocks about the size of golf balls. Once every 2 to 3 months pull the pump and basket out and rinse off the gunk with a hose.