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Koi Pond Filters

Many newcomers to Koi Keeping imagine that the function of pond filters is to obtain clear water in which to view their fish. In fact, the clarity aspect is of secondary importance. The primary object of the water filters is to enable the pond to support a greater number of fish than would be possible in a unfiltered pond, by removing the toxic end products of the nautral bodily process that take place within the fish. The bigger the filtration system is, the higher the stocking density that can be achieved. Even a dozen large Koi occupying a relatively small pond will, in the course of 24 hours, excrete a considerable amount of waste.


Pond Filters serve two purposes. It serves as a mechanical filter (removing dirt from the pond) and serves as a biological filter (providing the proper environment for the development of nitrifying bacteria).

Koi Pond filters are the most important piece of pond equipment you will need for your water garden and backyard ponds. If you are planning to have even the first Koi fish, do NOT treat this piece of pond equipment as an option. Fish are just like any other animal in that they excrete waste that would be toxic if it accumulated in their bodies. Fish are threatened if they are exposed to a buildup of toxic waste. You cannot over filter the pond. When buying your filter make sure you can backwash it properly or it's not going to work right. So the design of the filter has to be easy maintenance, and you have to have good flow of the water through the filter, so that all of your filter media and the bacteria is well oxygenated. The design and shape of the filter is important.

Ammonia and other toxic chemicals will quickly build up to a point where the fish will suffocate on their own waste, unless these toxins are continuously converted into less harmful products. Which is one of the main purposes of pond filters.

A pond is a completely enclosed water body that must deal with an abnormally high biological load. You will need to be aware of how to provide good water quality in an artificial pond, because if not your fish will be stressed and it will make them more susceptible to disease. Your filter needs to be big in order to keep your maintenance low and your water quality high.

Why do I need a Koi pond filter?

A Koi pond filter reduces the amount of ammonia in your backyard Koi ponds. You know (hopefully) that ammonia is the main cause of Fish death. Ammonia is a toxin and high levels can burn the gills, the skin, the gut and cause death.

Out in the wild fish are able to swim away when the water quality is not supportive to them, but in your Koi pond they are not able to swim away. This is why it is important to know how to care for your Pond and Koi.

What type of Filtration do I need?

Filtration systems are needed for two types of contamination, solids and chemicals.

The solids are materials not dissolved in the water, these are feces, uneaten food, leaves, and algae. The algae grows simply because there is an abundance of waste in the water. If you have a sufficient filter, you are able to funnel the waste out of the pond and drop it where it can be removed from the pond.

The chemical contaminates in a Koi pond are ammonia, dissolved organic carbons, nitrite and nitrate.

Filtration systems that perform both solid removal and chemical decontamination are what is needed for a Koi pond.

Basic Principles Of Pond Filters

1. All the water should pass through the pond filters system. Any water by-passing the filter will dilute the effect and efficiency of the filter system.

2. The entire pond volume should pass through the filter system as quickly as is practical and returned to the fish, i.e. the turn-over rate of the pond volume should be high.

3. Once in the mechanical or settlement part of the pond filters, the water should slow down. Slow moving water encourages settlement. Flow rates of the entire system must be maintained. Increasing or decreasing the size of the filter chambers can adjust the slowing down or speeding up of the water flow in the filter system.

4. In the bioconverter part of the system, the water can flow fairly quickly as the conversion of ammonia to nitrite to nitrate takes place immediately on contact with the nitrifying bacteria. The flow rate must not be too quick or the water may flush the heterotrophic bacteria from the media.

You need to place your submersible pump or bottom drain water pick-up (for external pumps) in the deepest part of the pond. That way you pull all the dirt and debris that has fallen into your pond out where the filter can filter it.

The type of filter system you use will make all the difference. If the filter does its job it will require some attention about once a week during summer season. If it never needs cleaning it is not doing the job of filtering.

Nitrogen Cycle

Fish secrete ammonia through their process of respiration and through urea. If left to build up, this ammonia becomes highly toxic to fish and will result in death. The process that converts ammonia into nitrites and then nitrates is the nitrogen cycle. Biological filtration provides a method to promote the nitrogen cycle and eliminates the harmful effects of ammonia and nitrites on Koi health. Because Koi are large fish (in terms of volume), they produce large amounts of ammonia and require adequate biological filtration to keep the nitrogen cycle performing adequately.

The nitrogen cycle takes about four to six weeks to become established in a pond. At this point, toxic chemicals can build up quickly, so very few fish should reside in the pond while beneficial bacteria becomes established. Once enough bacteria has established and is living on surfaces in the pond (as well as in filter media), more fish can be added. These bacteria will be able to convert toxic ammonia to nitrites and finally nitrates, which should be removed from the pond through plants and water changes.

Building a Ecosystem

Remember you are building an ecosystem not just a pond. Buying the cheapest water filters on the market leads to regrets down the road. If you don't have a pond filtration unit you will be setting your fish up for failure.

In order for the fish pond filters to be effective it must run 24 hours a day. The constant fresh supply of oxygenated water is what keeps the pond filter systems bacteria thriving.

A natural water body keeps clean by the constant flow of fresh water through it. In a pond water garden, a filter is required to keep the recirculating water clean and healthy for your fish, if they are deprived of this for even a couple of hours then they will start to die and the filter will lose its effectiveness.

Biological Filter

Unlike a swimming pool filter, which is to keep the water clear and clean, it is the chlorine added to the swimming pool which keeps the algae from growing on the sides of the pool and is really responsible for the clear water. In a Koi pond, we cannot use chlorine and we want the fine velvet coating of algae to grow on the sides of the pond.

In fact, if algae does not grow on the pond sides then there is a major water quality problem. Your Koi nibble on this algae, providing nutrients, giving them their luster and color.

We also want the water to be clear so that we can always see our Koi. Therefore something other than chlorine must be used to keep out the unicellular algae (causes pea-soup water) and the long stringy algae. These are the purposes of the Biological Filter.

Likewise, just because the water is clear does not mean that it is healthy for the Koi. In fact, the ideal water is clear with a slight greenish tint, as well as being odorless and tasteless.

A filter must be broken in gently over the first few months of its life. A vital part of filtration is living, unlike mechanical filtration, the bacterial colony takes time to become established. The steady turnover of water through the filter provides the bacteria with a constant source of “food” in the form of ammonia, as well as an essential supply of dissolved oxygen. Add a few fish at a time, so that the bacteria can adjust and catch up with the rate of ammonia being produced by the fish.

Power outages for 3-4 hours can deplete the oxygen in your filter and allow anaerobic bacteria to begin growing, aerobic bacteria (good bacteria) begin to die off.

When it comes to Biological filtration, you can never have too much filtration. Some state of the art Koi ponds have biological filters that are 10 to 20 percent the size of the pond. Even though adequate filtration is necessary for healthy Koi, impurities in the water build up over time; frequent water changes (10 to 15 percent per week) are helpful in eliminating impure water.

Mechanical Filter

Mechanical filters remove suspended debris or particulate matter from the pond. Most filtration systems remove surface debris through the use of skimmers or a settling chamber located at the start of the filtration system. As its name implies, a skimmer removes surface debris from the top layer of the pond. A settling chamber is an empty tank, usually situated before the biological filtration unit, that removes suspended matter before it is passed on to the biological filtration tanks.

Bog Filter

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

Ultra Bio-Media

Ultra Bio-Media is a sinking media with a very high biofilm surface that is 3 to 5 times greater than other plastic media, such as bio balls, various mats and foams, beads, ribbon and brush type media, etc. Superior bacterial adhesion and colonization is achieved due to the surface texture and grade of plastic used. Additionally, beneficial bacteria is able to build up into a bacterial matrix due to the excellent interstitial spacing within the media. Under a patent pending process, Ultra Bio-Media is seeded with bacteria and barley to kick start the nitrification process and to contribute to the control of string algae. When using a swimming pool filter on a water garden, the Ultra Bio-Media makes a great replacement for the sand. Sand filters are usually rated by how many pounds of sand to be used. 100 pounds of sand is about 1 cubic foot. So a 200lb sand filter could use 2 cubic feet of Ultra Bio-Media.

Ultra Bio-Media

Important Note:This unit requires an air pump for operation. This is not included with the item.

~Price $55.00 Individual pieces of media are approximately 1/4 inch in diameter by 1/2 inch long. Approximately 400 square feet of surface area per cubic foot of media.

Download your Free Ebook Click Here to download "The Three Step Process of Filtration"


How does your water become toxic?

Your Koi produce wastes:

Ammonia from their urine, which diffuses, yet the water remains clear and it becomes toxic to the Koi as it builds up solid wastes which also add more ammonia, plus toxic hydrogen sulfide, methane gas, etc. and it clouds up the water.

Other debris, as dirt,leaves, and uneaten food adds to the accumulation of toxins to the water quality.

Koi Pond Filters contain a biological material for aerobic bacteria to grow upon. These bacteria metabolize and rid the pond of the ammonia and wastes. Such a biological material in the 1980s would have been pea gravel or volcanic rock. Nowadays, however, many synthetic materials are being used (brushes, foam, or mattresses). The major requirement being that it contain the most surface area to house the maximum amount of good bacteria (aerobic).

Header Ponds

If you don't want to overload your pond with plants but want to have the healthy ecosystem by keeping plants in your pond then a Header Pond is the perfect solution.

A header pond is a smaller pond that feeds the waterfall that flows into the primary pond. Usually a header pond is constructed above the primary pond and is filled with many plants that have extensive roots.

Water hyacinth is a good plant to place in your header pond. Water hyacinth has many roots that act as a natural filter. This plant can cover your pond during the summer which is why having it confined to the header pond is a great option for using this plant.

Seeding the Biological Filter

If the pond or filter is new or if the biological filtering system has crashed one must basically start from scratch to seed his filter. Below are listed ways to kick start the growth of nitrifying bacteria.

Make Sure the Biological Filter is Big Enough — One can never have too much filtration. The heavier the fish load the more filtration is needed. If the filter is not adequate it will need constant cleaning and will not do a good job of housing enough bacteria.

Add Enzymes — Some enzyme products are better at helping build up the bacteria than others. MicrobeLift PL is our favorite for encouraging the growth of nitrifying bacteria while MicrobeLifts Super Start Bead Filter Bacteria is wonderful for systems with bead filters.

Hold Off Cleaning the Filter — As long as the filter is not clogging up the water flow and until the nitrifying cycle is established do not clean the filter. If it must be cleaned use de-chlorinated water from the pond.

Turn Off the UV — While the filter is seeding unplug the UV. Green water may be ugly to some of us but it is not necessarily unhealthy. It is better to allow the nitrifying bacteria to establish well before turning on the sterilizer.

Do Not Feed the Fish — Do not worry. The fish may get mad but they don’t understand that it is for their own good. The more food the fish eat the more waste in the system to convert to fertilizer. Resume feeding when the tests for ammonia and nitrites come back negative for a week

Perform Water Changes — Test the water every day. Change the water (remember to de-chlorinate tap water) if the ammonia is over 1.2 ppm and/or the nitrite reading is over 2 ppm if using Ultimate. If not, change the water whenever there is any reading of ammonia or nitrite. Keep monitoring and making water changes as needed until the filter is seeded.

Thin Out an Overcrowded Pond — It is hard to make the decision of who stays and who goes but it’s for the good of all the fish. Give the extras away or build another pond for them.

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Filtration System Components

Pre-filter or settling tank - Used for solids removal, this is essentially a large container through which water flows very slowly, allowing heavier-than-water solid matter to settle to the bottom. The settled debris is then flushed away during cleaning.

Mechanical filter- Used for solids removal, this is a container which contains mechanical devices such as brushes, matting, shredded PVC, sand and/or gravel. One variation is a pressurized container that contains a screen or other filtration media such as small cylindrical or round (bead-like) materials.

Bio-converter- Used primarily for ammonia reduction; this is a container which contains high surface-area media that encourages the growth of large numbers of beneficial bacteria. Air is sometimes introduced to enhance the effectiveness of the media. These units may sometimes be pressurized.

Ultra Violet (UV) Light – Used to control floating or suspended algae (the algae that causes green water). Intense UV light can kill the algae.

Pond Filters Beginning

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