Pond Filter Basics

Find out the mechanics of the devices that keep your pond clean.

Koi pond with oxygenation. Via lindsayΔlachance/Flickr

There are two fundamental types of filtration: mechanical and biological. Both are available and utilized within the ponds and water gardens industry for ornamental water feature applications. Many people may think there are more than these two types of filtration, based on the inappropriate use of some terminology by a variety of industry manufacturers. We need to remember that a filter is a device that actually has the capability to clean the water.


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If it does not clean the water, but rather provides other water treatment processes, it is not really a filtration unit. An example of a nonfilter device would be an ultraviolet light or an ion-generating unit. These types of devices treat or alter the water through a variety of reactive processes, but still require the use of a filter to complete the removal of any unwanted debris and/or particles. Neither of the two above-mentioned devices are actually capable of removing anything from the water and therefore cannot be considered a filter.

Mechanical Filtration

If you bend over and pluck a floating leaf from the surface of a pond, you have performed a mechanical filtration action. This type of filtration physically removes debris and suspended particles from the water by using a variety of filter devices, surfaces and porous media materials, such as sand, brushes, pads or baskets. Water passes through these items and depending upon the coarseness of the material being used, the media collects the unwanted particles by not allowing the debris to pass through.


The debris enters the filter. Photo by Rick Bartel

Typically, mechanical filtration units incorporate several types of media or other surfaces that selectively collect varying sizes of debris in different stages. An example of this type of filtration would be a skimmer, using a collection basket or net to collect larger debris and then having the water flow through a media pad that collects smaller debris before the water enters a pump system. Other examples of mechanical filtration are bead filters, sand filters or pre-filters found on some external pumps.


The large debris is captured by the filter basket. Photo by Rick Bartel

The two items that allow a mechanical filter to function properly are the size of the available pores in the filter material which determines the size or type of the debris being removed, and the volume of water that is forced through this media. In the case of skimmer filters, the two items allowing proper functionality are the size of the weir opening or inlet available for inbound water flow, and the size of the pump pulling the water through the filter media.

With skimmers, the most frequent or common error committed by pondkeepers and contractors alike that results in a nonfunctioning filter unit is not properly sizing the skimmer’s weir opening to handle the appropriate flow of water without breaking the cohesive bond of the moving water molecules. If you draw too much water too quickly into the skimmer body, the cohesive bonds between neighboring water molecules are broken and you end up sucking mass quantities of water into the filter without pulling the debris along with it. It is imperative that the water be pulled slowly and gently into the skimmer, dragging all of the floating debris and particulates, to be collected inside the filter for later disposal.

The small debris is captured in the filter mats or brushes. Photo by Rick Bartel

It is essential that professionals within our industry know about the different types of filtration devices and how to select them for use within the varying types of water features. They should know how these filtration systems actually work, so they are properly installed. If a filter cannot do its job, the resulting problems create significant reasons why the general public chooses not to have water features installed on their property. No one wants a green water pond!


An overview of the path of the flow of water through a skimmer filter. Photo by Rick Bartel

Biological Filtration

This type of filtration differs from mechanical filtration in that it does not collect and remove or otherwise separate the debris from the water. Rather, it consumes the unwanted particles by way of using natural processes from naturally occurring beneficial bacteria. Beneficial nitrifying bacteria are found naturally occurring in all living bodies of both fresh and salt water. These bacteria are known for consuming large quantities of excessive nutrients in the water. If left unchecked, these nutrients can result in extremely unstable water quality issues.


The components inside a water filter work together to remove debris from the water.

The only two items that allow a biological filter to function properly are the amount of surface area available for colonization of beneficial nitrifying bacteria, and the type of filter media used that will allow the bacteria to easily adhere to its surface. The more bacteria housed within the biological filter, the more efficient the filter can become in removing unwanted substances from the water. Beneficial bacteria more readily adhere to carbon-based surfaces found in many filter ribbons, pads and mats or natural media such as lava rock and sand. For this reason, manufactured plastic bio balls never really worked and are rarely used in today’s market, as the bacteria had extreme difficulty adhering to the slick plastic surfaces.

The pore size or openings manufactured into any filter material will ultimately determine the size of debris being removed from the water. Photo by Jessica Pineda/Courtesy Laguna Koi

Over the years, the lack of proper use of both mechanical and biological filters has resulted in the false general consensus that certain installation techniques should not be used in order to keep ponds clean. A great example is the use of rocks and gravel on the bottoms of ponds. Many people falsely believe that having gravel or rocks on the pond bottom allows excess dirt and debris to build up on the bottoms of pond applications when in fact the real problem is an inadequate filter. If a properly sized filter is in place, there will be no debris build up. If there is debris, it is obvious that an appropriate filtration unit is not in place. The bottom line is if there is dirt and debris in a water-feature system, then there is a  need for better filtration!

Proper Installation Tips

Always install all filters according to manufacturers’ specifications. Though there are many varieties of available filters on the market, and most are similar to one another, no one knows the design of these filters and how they were intended to be used better than the manufacturing company. This will also protect the contractor or consumer if something should go wrong with the unit. One of the very first things a manufacturer will ask when discussing the warranty of their products is if the product was being used as intended and if it was properly installed by a qualified person.
Everyone typically likes to take photographs or digital images of their completed water feature and homeowner-owned photographs of the water feature project have many times shown that the failing operation of the filter in question was indeed improper installation. Following the manufacturers’ specific recommendations for installation protects the homeowner and the water-feature contractor under the product warranty for that particular piece of equipment and ensures that it is being used properly and as designed.

Hide the Filtration Units

There are several appropriate installation techniques and methods available to water feature contractors, enabling them to better meet their project goals when it comes to the installation of necessary components within the landscape. I use the Random, Irregular, Spontaneous and Erratic Method (R.I.S.E. Method), which I wrote a book about in 2010. It uses natural materials to not only camouflage these needed components but to also make the overall appearance of the water feature blend more satisfactorily into the surrounding landscape.

After your filter is installed, you can hide it within the pond’s landscape. Photo by Jessica Pineda/Courtesy Laguna Koi

Every water feature contractors must adequately hide and camouflage these necessary but unsightly man-made devices. No client wants to look out into their landscape and see pumps and plumbing, or filters and valves cluttering their view. Qualified contractor should be able to hide these devices within the parameters of the customers’ landscape, hiding them from view while still allowing easy access for routine maintenance issues.

The best way to hide these devices is to simply stop placing them directly in front of the homeowner’s face. Though this may sound a little obvious, it is the leading reason why these components can be seen so easily. Many projects I have personally visited over the years have clearly shown that the ill-thought-out location of component placement is the No. 1 reason for an unattractive end result.

Many times contractors tend to get far too complicated when trying to hide water feature components and typically the simple answer can be one of the best solutions. This doesn’t always mean that the item needs to be buried and completely camouflaged from every possible angle. Sometimes the item just needs to be sitting behind a well-placed bush or shrub, hidden from the field of view where the homeowner or visiting guests spend the majority of their time when viewing the water feature.

Some manufacturers offer components that are designed to sit off-site, (such as Savio’s F-100 and F-200 series biological filters), where they can be more easily hidden behind a decorative fence or around the corner on the side of a house, or under a deck, etc. Design options such as these will certainly give the contractor or homeowner a more diverse set of options to consider when attempting to hide these necessary filtration devices in the landscape.

Remember, the success of any ornamental water feature requires the proper use of the appropriate types and sizes of filtration units, adequately hidden within the landscape, leaving a pristine view to be enjoyed by all who happen by.


A 30-year experienced veteran master contractor and two-time water feature contractor of the year, Rick Bartel is currently employed with Atlantic Water Gardens as the director of international sales.

Article Categories:
Fish · Ponds and Koi