07. Filtration biofilters for koi goldfish ponds.

Installing a bio filter is the price we have to pay for wanting to keep koi in an artificial pond. It performs the functions that occur naturally in a lake or river within a compact unit and is arguably the single most important factor that makes keeping koi in an artificial pond a possibility.

Over many thousands of years, aquatic ecosystems across the globe have reached an equilibrium, where the number of producers and consumers is balanced, each exploiting a niche within a most intricate web of interactive relationships.

Relationships within a natural environment develop that determine the amount of energy (or food) that flows through the food chain, each link in the chain balanced in a sustainable relationship with the next. In this way, a natural aquatic environment is able to support a limited number of fish through food it provides them and the amount of waste it is able to handle. And then, overnight, we want to break nature’s rules by stocking an unnaturally high density of fish in a relatively small pond and feed them more food than Mother Nature could ever sustain. – Enter the need for a filter in the artificial koi pond.

Koi pond filtration – what is it?

Koi pond filtration is a series of complementary processes that act on water to produce a high quality, pollutant-free product. Rather than compare pond filtration to other forms of filtration (such as air, oil and smoke filters where materials are physically removed), it is perhaps more accurate to describe pond filtration as reprocessing, where in most cases the contaminants are not removed, but transformed within the filter into less toxic by-products.

A koi filter is designed to carry out the various filtering and reprocessing functions in a compact area that would routinely be carried out naturally in much greater volumes of water. The tasks that a koi filter is required to carry out are determined by the natural processes of a lake or river, and consequently, a koi filter must embody the same number of processes. If it doesn’t, then the koi’s environment will suffer, leading to koi health problems.

The tasks that a filter must carry out.

1. The most important role for a filter is it’s biological function. If a filter is fitted with suitably porous and stable media and fed with a continuous supply of polluted water, then bacteria will soon colonise it. These bacteria colonise the media naturally, within the water flow, attracted by the soluble waste products from which they gain their energy. These are the same bacteria but will colonise a lake or river, thriving in similar niches but within a koi filter. They will be found in a much greater density, reprocessing significantly more waste. These bacteria break down the toxic soluble ammonia that is excreted by koi that would lead to water quality problems if allowed to accumulate in the pond. The bacteria can take months to become fully established so that they can cope with waste levels produced by a heavily stocked pond. This is the most vulnerable period for koi, and is the time when most water quality problems will arise. But over time, a diverse population of autotrophic and heterotrophic bacteria will colonise the filter media, producing improved and stable water conditions. Some of these bacterial colonies will convert ammonia into nitrites and then into less toxic nitrates under aerobic (well oxygenated) conditions.

Even though biofiltration is the only type of filtration required to keep koi healthy in an artificial pond, it is routinely pre-ceded by mechanical filtration in typical koi pond systems. – But why?

Mechanical filtration serves two purposes in a koi pond.

Aesthetics. Mechanical filtration is the first type of filtration in a koi pond (and is sometimes called primary filtration for that very reason). It removes suspended debris from the pond by trapping or settling particulate matter. However, this is carried out largely for aesthetic reasons, so we can see our cherished and spectacular koi. Koi themselves do not actually require clear water (and in fact thrive in the murky conditions of a mud pond).

Filter function. Even a small filter system will handle several thousands of gallons of water every day. Mechanical filtration plays a key role in preventing solid matter from gathering in the biological chambers. If silt or other sediment was allowed to settle within the fine structure of the biological media, then the biological function of the filter would deteriorate, leading to water quality problems. It is vital that biomedia be kept as clean and unblocked as possible to allow a high-density of bacteria to populate the media. As soon as settled debris is allowed to create dead spots within the biomedia, then traces a toxic ammonia and nitrite it will start to accumulate in the pond water, having a knock-on effect on koi health.

UVcs. Again, it is for our benefit (rather than our koi’s) that a koi filtration system incorporates a UVc. Installed in-line, a UVc irradiates single celled algae suspended in the water flow, causing the cells to clump together into larger more filterable particles. This prevents green water from forming (which incidentally koi absolutely thrive in) and allow you to view your koi in crystal-clear water.

Once you have embarked on a crusade for crystal clear water, there is no turning back and water that is ‘nearly’ clear will not do. Experience shows that more consideration must be given to the mechanical side of filtration compared to the biological function, as this often becomes your filter’s limiting factor in producing the environment you require. Consequently, you should plan or opt for a filter with significant or even ‘over-sized’ mechanical and primary capabilities. This also includes a system that is easy to maintain, making it more likely that you will carry out regular maintenance.

Why are there so many different types of filter?

Having established the key components that make up an effective filter, there are a limitless number of designs that can execute the functions of a filter. The most conventional (and practical) filter design is a multi-chambered approach, where chambers are clearly designated for different roles. Variations will occur between different off-the-shelf filters designating different ways of carrying out the mechanical and biological functions.

Methods for mechanical filtration are as numerous as the number of different types of media available for biological filtration.

Methods of mechanical filtration.

Settlement and vortex. These methods use subtleties of water flow within the chamber to settle out suspended particulate matter. When the speed of flow drops, water’s ability to keep solids in suspension also drops, resulting in the settlement of solids in the chamber.

Entrapment. This method is used in standard external black box biofilters, acting in a similar way to a sieve. In a multi-chambered system, methods of entrapment are varied and include hanging brushes in a primary chamber through to the self-cleaning microscopically fine mesh of ‘The Answer’. Sand pressure filters use a bed of silica sand to trap solids, but unlike other methods, must be pump fed.

Biological filtration.

Biological media must have features that lend themselves to being colonised by beneficial bacteria in flowing water. It must also be relatively easy to rinse and clean periodically. 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 we have 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. However, unlike mechanical filtration that can prove to be very limiting for a pond’s performance, I have rarely heard of a pond’s biological performance suffering from the incorrect choice of biomedia.

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.

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 multi-chambered filter will sit adjacent to the pond, situated at the same level, so the water level in both pond and filter are the same. Water leaves 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. 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 greater velocity. Also, unlike a gravity fed filter, a pump fed system will 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.

Golden rules.

Always try to provide as much mechanical filtration as possible as this tends to become the limiting factor in providing clear and stable pond conditions, especially if a UVc is installed.

A filter must be given the opportunity to mature, allowing the filter bacteria to colonise and cope with the levels of waste produced by your koi. Stock a new pond gradually, and check in the first three months that ammonia and nitrite readings are zero.

When calculating the size of pump you require, make sure that it will pump the volume of your pond through the filter at least once every two hours.

Beneficial filter bacteria are aerobic and require a steady supply of oxygen. Consider adding strategically placed air stones in your biofilter to enhance their performance.

Effective mechanical filtration will soon collect significant quantities of solid organic matter. Remember to completely remove this from your system by regular cleaning as it still puts the burden on your pond’s DO levels even when trapped in the filter.

Kill blanketweed and string algae.