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You are here: Home > Build a Koi Pond > Pond Filters > Bog Filters

Bog Filters



For Clean Water Think Plants!

Bog filters are a natural and beautiful way to make your pond water clear and healthy for your Koi and Pond fish. A bog filter will not only help in your pond cleaning chores but think of the money you will save.

Since your filtered water will be very rich in nitrates, a bed of "greedy" aquatic plants will consume a great deal of this before the water returns to the pond.

At least 10 percent of the pond surface should be dedicated to a plant filtration area.

Plants good for a Bog Filter:


water lily pictures

This Calla Lily will do well in a bog filter because its roots can thrive in very wet conditions.
The roots absorb wastes in the pond water, and these wastes contain nutrients that promote spectacular growth.



  • Arrowhead (Sagittaria latifolia)
  • Black taro (Colocasia esculenta "Black Magic")
  • Bog lily (Crinum americanum)
  • Caladium (Caladium x hortulanum)
  • Calla lily (Zantedeschia)
  • Pretoria canna (Canna x generalis 'Pretoria')
  • Corkscrew rush (Juncus effusus spiralis)
  • Daylilies (Hemerocallis)
  • Forget-me-nots (Myosotis scorpioides)
  • Joe-Pye weed (Eupatorium fistulosum)
  • Louisiana iris (Iris fulva 'Louisiana Hybrids')
  • Cardinal flower (Lobelia cardinalis)
  • Sweet potato vine (Ipomoea batatas)
  • Meadow rue (Thalictrum)
  • Papyrus (Cyperus papyrus)
  • Dwarf papyrus (Cyperus isocladus)
  • Variegated striped rush (Baumea rubiginosa 'Variegata')
  • Red-stemmed sagittaria (Sagittaria lancifolia 'Rubra')
  • Swamp milkweed (Asclepias incarnata)
  • Sweet flag (Acornus calamus)
  • Ribbon grass (Phalaris arundinacea)
  • Society garlic (Tulbaghia violacea)
  • Red-stemmed thalia (Thalia geniculata 'Ruminoides')
  • Trilliums (Trillium grandiflorum and others)

As water flows through plants, roots and stalks add oxygen, and remove ammonia, making the water clearer.

Some plants can be too aggressive for your bog. Here are plants to avoid are cattails, mint, chameleon plant, horsetail, and yellow iris. (This is only a couple of invasive plants, so make sure you read up on the plant before planting it.)

Plants such as giant papyrus, umbrella plants, cypress and taros are all great plants for your bog filter because they can absorb a lot of waste from the pond water.


Tips to remember when building a bog filter:

  • Make sure it is big enough. Water gardens it needs to be 10 - 15 percent of the surface area, if you plan on keeping koi it needs to be bigger 25 - 30 percent.
  • Do not make the gravel bed too deep, 12 inches of gravel substrate will do.
  • Use three-eighths-inch pea gravel.
  • Initially plant one plant per square foot, and do not wash the soil off the roots before planting, just take the plant out of the pot and plant it directly into the gravel.

You will not need a pre-filter on your intake pump with a bog filter. If you use enough plants they will be more than capable of keeping your pond clean.

The Way It Works

A pump pulls water from the pond and sends it through a PVC pipe toward a shallow bog. The water gently washes through a bed of gravel planted with a variety of bog plants. Beneficial bacteria that thrive in this environment convert toxins (created by fish and plant waste) into nontoxins, and the plants grab the nutrients that would otherwise feritilze algae. The cleansed water spills back into the pond, for a touch of creativity you can have the clean water go back in through a waterfall.



Common Errors to Avoid when Installing A Gravel Bog Filter

  1. Bog too small. For water garden, 10 to 15 percent of the surface area should be bog; for Koi ponds, the percent should be increased to 25 or 30 percent.
  2. Gravel bed too deep. You need no more than 12 inches of gravel substrate. If adding a gravel bog to an existing deep pond area, you should construct a false bottom by using grating.
  3. Wrong-sized gravel. Use three-eighths-inch pea gravel.
  4. Not enough plants. An initial planting of one plant per square foot.
  5. Wrong plants. Aggressive species can overrun your bog. Think twice before planting those that can be invasive, such as cattails, mint, chameleon plant, horsetail, yellow iris and others.
  6. Washing soil off the roots before planting. There are not enough nutrients in a new bog to sustain new plants. Just knock the pot off the plant and plant it, soil, roots and all, directly into the gravel.
  7. Not taking plants out of their pots. Pots severely limit a plant's ability to absorb nutrients, and they defeat the purpose of the gravel bog filter.
  8. Starving the bog. Don't place a pre-filter on the intake of your pump. It not only stresses the pump, but it traps nutrients that the bog plants need to survive. This refers to a true mechanical pre-filter (usually made from foam pads, which need frequent cleanings), and not to a pump protector or intake screen, which is recommended.

Resources: Anita Nelson, co-owner of Nelson Water Gardens




For information on a Biological Filter Click Here

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