pond filter options

A pond filter is the inseparable partner of the pond pump that we looked at last month. Just as the pump’s role can be compared to that of our heart, the filter’s role is that of the liver and kidneys combined. All of the water that flows around you pond must at some stage flow through your filter. It’s here that the storage and reprocessing functions take place, making sure that both solid and soluble impurities are removed, broken down or retained so they don’t go on to harm the whole pond. So the liver and kidneys that we choose for our pond are really a matter of life and death. We need to consider buying a filter for our pond because of how we choose to keep our koi. Rather than being lightly stocked as nature may have intended, an artificial koi pond is heavily stocked, requiring our intervention to maintain a balanced environment. So now that we need a filter – how do we go about choosing the right one?

Given that a filter’s role in every pond is essentially the same, you might expect there to be a single type of filter that suits all. But in the same way that there are hundreds of different models of car to choose from (and yet they all essentially get us from A to B), there are equally many different types of filter to choose from. There is a collection of many different pond-specific factors that we need to consider when buying a filter for our own pond.

If you want the best pond filter for your water pond and fish, see these ones…

Pump fed or gravity fed?

Irrespective of the apparent limitless variations in filter design there are generally only two methods by which water is fed into a filter. Each method will affect the options and performance of the filter concerned.

Gravity fed

This is the preferred method in koi filtration – especially for larger ponds. The filter (which is usually multi-chambered) will sit adjacent to the pond, situated at the same level, so the water level in both pond and filter are the same. Water exits the pond by gravity either through a bottom drain (s) or skimmers (or both). This dirty water enters the mechanical chambers at a relatively slow rate, aiding settlement (or whatever the method of solids removal your system uses). The water then passes through the biochambers and into the final pump chamber where a pump (usually submersible) returns the water to the pond, perhaps through a Venturi or via a waterfall. In this way, the water is effectively pulled through the filter system.

Pump fed

Perhaps most of us are more familiar with pump fed filters because of their ease of installation – typically used in a pond keeper’s first pond. Pump fed filters tend to remove particulate matter by entrapment as the water enters the filter at a high velocity. Also, unlike a gravity fed filter, a pump fed system will usually sit higher than the pond, returning water via gravity after it has passed through the filter. These filters are rarely multi-chambered, and consist of a single chamber containing both mechanical and biomedia. Because these filters are situated above the waterline, they are sometimes described as trickle or ‘dry’ filters, compared to the gravity fed ‘wet’ filters whose media is permanently submerged in water. Pump-fed pressurised filters are a relatively new method of filtering, meaning that they don’t need to be situated above the water level, but can be buried and hidden out of view. However, they tend to be less suited to larger volume (>2000 gallons) koi ponds because of their limited volume and filtering capacity.

What should I be looking for from a filter that’s going to be used for an average 2000 gallon koi pond?

Perhaps the most useful starting point is to look at a multi-chambered filter as a bench mark in koi pond filtration. It has long been the favourite for koi keepers because of its performance and flexibility. As you would expect, new methods and concepts of filtration have been introduced to the hobby over recent years, offering benefits and enhancements compared to the industry-standard multi-chambered gravity-fed filter. To consider any filter option, it should be compared to that standard.

I suggest multi-chambered filters have been (and in many cases, still remain) so popular and effective in koi pond filtration because they tick so many boxes that any filter you choose must do. If you feel that an alternative, more modern method of filtration may be more suitable to your particular pond, then you should always judge it’s performance against the multi-chambered filter.

1. Filter performance and maintenance

Modular, compartmentalised design and function.

A filter’s performance is a function of it’s design, it’s size in relation to the pond, the flow rate through it and the media used within it. The modular design of a multi-chambered filter demonstrates beautifully the roles that any filter that you consider buying for your pond should fulfil. Opinion and preference play significant roles in deciding which filter to buy. At every stage, ask questions such as:

What are the benefits?

Why is that feature important?

What advantages does model x have over model y?

Is that feature significant in my situation?

This is particularly true when examining how your potential filter purchase actually works.

1. Mechanical.

Settlement and vortex.

These methods are used in a gravity-fed system and use subtleties of water flow within the chamber to settle out suspended particulate matter. When the speed of flow drops, the water’s ability to keep solids in suspension also drops, resulting in the settlement of solids in the chamber. These methods usually require considerable volume (and therefore space) to achieve good results.


In a multi-chambered pump-fed system, methods of entrapment can vary. This may involve layers of foam or brushes in standard filters through to the more advanced self-cleaning micromesh screen as found in ‘The Answer’. This method is also used in standard pump-fed external black box biofilters, as well as more innovative bead filters, acting in a similar way to a sieve.

Before buying any filter, always assess its mechanical capabilities. Mechanical filtration is an area of filtration that we can easily overlook as being important. Not only will inadequate mechanical filtration lead to frustratingly poor water quality, but also to a limited biofiltration performance as your filter matures through the years.

The maintenance factor.

When considering the efficiency of a mechanical filter you will have to make a compromise. In theory, it should be relatively easy when designing a mechanical filter to build it so that it removes debris very quickly and down to as small a particle size. But the filter’s ability to remove even the finest particulate matter must be balanced by how frequently you are happy to clean or maintain that filter. So when choosing a filter, you should strike a balance between filter efficiency and performance and your own maintenance needs (i.e. how often you have to roll your sleeves up to work on the filter). If maintenance was not an issue, then we would choose ultra-fine pore filtration systems that intercept the finest of particles, but would need cleaning or backwashing on a daily basis! Each chamber in a multi-chambered filter should have it’s own drain to waste so that they can be drained independently. Remember to check the maintenance features of any filter before buying as it will save you hours in maintenance time.

2. Biological Filtration

The vast majority of filters are supplied with media. It is unusual for a correct-sized filter, fitted with suitable bio-media and provided with the correct turnover not to meet it’s biological expectations. I think we tend to forget that bacteria readily colonise on every hard surface within the whole pond system and consequently do a good job at reprocessing pond water. The largest hurdle to overcome when managing the biological side filtration is allowing sufficient time for the filter to become alive and mature.

But if you are choosing media yourself, biological media must have features that lend themselves to being colonised by beneficial bacteria in flowing water. Your choice of media must also be relatively easy to rinse and clean periodically (back to low-maintenance filtration again).

Several decades ago, gravel was the preferred biological media, but as we have gained a better understanding of the requirements of bacteria, and where technology and the introduction of new materials has allowed, the nature of biomedia has changed dramatically. Long gone are the days of gravel (even though many pond keepers enjoyed success with it) and the market has witnessed the introduction of a vast array of new biomedia ranging from open cell foam, Japanese matting, flo-cor, Alpha grog, porous glass media and even plastic packing tape. New types of media boast better surface areas and easier maintenance than the last.

2. Space.

Each filter will be rated to a pond’s volume and a flow rate. This will then help you to determine how much space you will need for your filter. Alternatively, perhaps your choice of filter will be determined by the space you have – in which case a filter’s footprint will be very important. Don’t forget to take your floor plan with you when investigating the filter market.

3. Cost.

Wouldn’t koi keeping be great if cost was not an issue! Unfortunately, we all have a budget of how much we are willing to spend on a pond, and part of that budget will be allocated to a filter. Be sure before you embark on a pond project that you have sufficient funds to fit out a new pond adequately. Skimping on filtration or any other vital component of a pond system will lead to problems in the future. Before your spade cuts the first turf, you should already have done your research into what type of filter will suit your performance, maintenance, space and budget requirements – because once you’ve started – there’s no turning back!

Research before you buy

This brief article is designed to give you pointers and help you find the right questions to ask – the rest is up to you. Koi keeping clubs and societies are an ideal source of experience and expertise for further research. Members are more than keen to share their experiences and opinion and may even offer to show you around their pond. The more ponds you can see and experience will not only make your pond and filter system better, but will also save you time and money. It is the tips and trade secrets gleaned from conversations that make a good system truly excellent.

The factors that will influence the performance of a filter

1. Mechanical efficiency. How much solid matter can be removed and retained in the filter and down to what particle size.

2. Maintenance. How frequently will the filter need cleaning? How easy is it to clean?

3. Biological action. Will the media support the bacteria required to sustain your pond full of koi – with capacity to spare – just in case?

4. Installation. How easy is the unit to install?

5. Footprint. If size and location are real issues – how do the different filter options compare?

6. Cost. The filter will sustain valuable koi for many years. Are differently-priced options comparable in performance?

Box-out: Filter-types

Pump-fed. A submersible pump in the pond pumps water from the pond up to an external filter. The filter may be a trickle filter, returning to the pond under gravity, or pressurised, returning to the pond under the pump’s pressure.

Gravity-fed. Water is constantly being pumped out of the final chamber in the wet filter, causing the water level in the external filter to always be lower than that level in the pond. The water leaves the pond via a bottom drain to the external filter via gravity, thereby completing the cycle.

Box-out. Top features

1. Easy to clean. The more automated the better (making it more likely that you do it!).

2. Effective solids removal

3. Compact and easily concealed

4. Robustly built and fabricated to last

5. Large biological capacity

Kill blanketweed and string algae.