A veggie filter, sometimes known as a bog filter or plant filter is a natural garden pond biological filtration method that uses aquatic plants as a means to remove impurities from garden pond water.
A veggie filter is home to lots of pond plants that are deliberately starved of nutrients.
In order to survive they absorb the nutrients directly from the pond water.
Once the pond water containing ammonia and nitrates has passed from the pond into the veggie filter and then back into the pond, the impurities have been removed.
What Size veggie Filter Do I Need?

• Calculate Pond Volume - Length x Width x Depth e.
  g.
  10ft x 10ft x 10ft = 1000 cubic feet.
  
• Convert to gallons by multiplying by 7.
  48 e.
  g.
  1000 cubic foot x 7.
  48 = 7480 gallon.
  

Now that you know your actual pond volume in gallons you will need to build a veggie filter that is at least 10% of the volume of your garden pond.
If your pond is 1000 gallon then you need a Veggie filter that is 100 gallon; a veggie filter that is 150% is better if you have a lot of fish in your pond, as it will remove impurities at a quicker rate, making for a healthier pond environment.
Some pond plant species are more suitable for use a veggie filter than others, depending on factors such as climate, hours of sunshine, growth pattern and where the veggie filter is located.
Leafy aquatic plants that grow quickly are usually a good choice.
The following pond plants are a selection of recommended varieties that are commonly used in veggie filters:

• Azolla, also known as Fairy Moss grows extremely quickly and is capable of removing large amounts of nitrogen and carbon through photosynthesis.
  
• Duckweed, classified as a floating pond plant is commonly used and also acts as an occasional food and dietary supplement item for your goldfish and koi.
  
• Water cress is well known for being a plant that consumes high levels of nitrate and would therefore be an excellent choice for your bog filter.
  
• Soft rush / Pickerel rush / True bulrush or Flowering rush grow quickly for maximum biomass and are excellent for use in most bog filter applications.
  
• Arrowheads are yet another species of aquatic pond plant that are ideal for your veggie filter.
  
• The Norfolk reed, classified as an oxygenator not only removes pollutants but because it's leaves are submerged below water allows oxygen to dissolve into the pond water and not into the atmosphere.
  
• Pennywort, Water Hyacinths, Sweet Galingale and Water lettuce are popular choices too.
  

Veggie Filters Advantages:

• A veggie filter does not incur electricity costs, unlike a biological pond filter that requires mains power to operate.
  
• As there are no electrical components used in a veggie filter system there is no chance of electrical mishaps and potential harm to aquatic pond life.
  
• A veggie filter fits in with the life style of those pond keepers who believe that letting nature take its course, rather than using man made inventions is the solution.
  
• The aquatic plants act as supplemental food source for the fish living within the pond.
  
• A veggie filter effectively removes both suspended algae and filamentous string algae varieties such as blanket weed, sometimes referred to as pond scum.
  The veggie filter plants compete directly with the green water and blanket weed for nitrates in the water.
  Traditional biological pond filters control algae blooms but are unable to remove blanket weed that sticks to the sides of your pond and almost any other available surface within the garden pond.
  
• The attractive foliage and plants contained within the veggie filter add a touch of beauty and serenity to backyards and outdoor living areas.
  A traditional pond filter, although extremely efficient is somewhat of an eyesore and usually needs to be disguised or hidden from view.
  

Veggie Filter Disadvantages:

• Blocked and tangled pump impellers due to stray roots and foliage.
  
• A messy pond environment due to fish eating and damaging the plants.
  
• A veggie filter will require a lot more maintenance and care on an ongoing basis: diseased and dead plants will need to be removed before there is an opportunity for the leaves to decay in the water and increase the levels of ammonia in the pond system; aquatic plants will have to be cropped back periodically; plants that have uprooted will need to be replanted etc.
  
• Bacteria and parasites live on plants and by introducing these into the pond you are in effect introducing potential problems.
  

A veggie filter bog filter needs to be thoroughly cleaned at least once a year.
This is usually carried out in the fall (September in the Northern Hemisphere).
You need to remove all aquatic plants and clean them.
They also need to be trimmed, tied back and divided before being placed back in the veggie filter.
If you live in a cold climate you may want to store the plants over the winter months.
Every good veggie filter should contain a bottom drain for clearing the waste and this should be opened every two to three weeks to allow sludge and other organic matter to be removed.
To summarize I would like to add that although a veggie filter is the preferred choice of pond filtration method for some pond keepers and water gardeners, it is less hassle for the vast majority of those new to pond keeping to use a traditional garden pond filter as they are relatively maintenance free, easy to install and readily available.