U & V: Pond Keeping Terms – Undergravel to Vortex

Undergravel Filtration

A method of filtration that is perhaps best-suited to aquaria where maintenance is quite straight forward. The principle of undergravel filtration in ponds has to some degree been superseded by multi-chamber external filters, which can be maintained very effectively.

Underground filtration relies on a bed of gravel performing mechanical and biological functions, as water (and debris) is drawn down through the gravel bed by a submersible pump.

At the base of the bed of gravel sits a network of perforated pipes, which are connected to the suction end of the pump. As the pump draws water, dirty water is pulled through the bed of gravel.

Advantages of this method of filtration are that it is out of sight and can produce very stable and clear water conditions. However, it uses up valuable pond space, is impractical with inquisitive koi and will require a complete spring clean every 2 years or so. They also have a reputation for blocking.

Unionised Ammonia

Ammonia is found in two forms; NH3 (unionised), and the ammonium ion NH4+. The ammonia that is excreted by koi will adopt one of the two forms depending on the chemistry of the pond water. As pH and temperature rises, so does the ratio of the more lethal NH3. Irrespective of water chemistry, it must be stressed that zero ammonia reading must be the aim of every koi keeper.


An ulcer is a localised area of dying tissue found on the exterior of koi and other pond fish. They first appear as a small area of reddened scales, which begin to lift away from the body, eventually dying and dropping off the fish. The score or ‘ulcer’ that remains can develop even further, and if unsuccessfully treated, can ultimately cause the affected koi to die.

Ulcers are caused by virulent and aggressive bacteria that are commonly found in all ponds. These opportune bacteria manage to infect a koi after that koi has become stressed through poor handling, and water quality or specific bumps or bruises.

Koi that are suffering from ulcers will rarely show a behavioural change (which is quite unique when considering the impact of other diseases on koi). Where external parasites may cause fish to flash and scratch or bacterial gill disease may cause koi to gasp, koi suffering from ulcers will usually continue to swim, feed and behave like every other fish in the pond. In fact this can be a frustrating characteristic with ulcers, in that they appear to be quite random in their occurrence, appearing on perhaps the most apparently healthy fish in the pond.

Ulcers were the scourge of the nineties, with shipments of koi regularly developing ulcers, that despite the best efforts of medications that had once been reliable and effective against all other infections, these appeared to be completely ineffective against ulcers. This is largely down to the bacteria’s abilities to resist traditional medications.

A recognised method of treating an ulcerated koi is to adopt a 3-prong approach which both supports the koi and treats the bacteria:

1. Add salt to the pond.

Salt reduces stress by reducing water flowing into the open wound, reducing the workload of the kidneys which excrete the water. The salt also acts as a broad-range disinfectant, reducing bacteria in the pond and on the fish.

2. Topical treatment of open ulcer.

Antibacterial poultice mixes can be applied or sprayed onto the open wound, reducing the localised invasive infection.

3. Antibiotic injection.

It is believed that the majority of ulcer-causing bacteria are antibiotic-resistant, leaving them difficult to treat through the use of prescribed antibiotics. However, a vet, who has a knowledge of the trends of antibiotic resistance in koi can offer a best-guess, or can take samples to determine the susceptibility of the bacteria to a range of antibiotics. As a koi suffering from ulcers will also be suffering from septicaemia (blood poisoning), then antibiotics that have an effect on the whole body are required.

The above 3 actions, accompanied by a good water quality and a balanced diet will give your fish the best chance of recovery.


Ultra violet light is a form of invisible radiation that is found between the visible blue end of the spectrum and X-rays. It covers a broad band ranging from 13nm to 400nm, with the germicidal UV light being most effectively at 254nm. (Other UV spectra can be found in sun-beds, counterfeit note detectors and disco lights). UV light at 254nm is an indiscriminate living tissue killer, killing both pathogenic and non-pathogenic cells. (That’s why our skin peels after unprotected sun-bathing). It also causes single-celled algal cells to flocculate (stick together), enabling them to be removed by mechanical filtration.

UV’s are generally accepted as a natural addition to a filtration system, with many box-biofilters for smaller ponds being supplied with an integral UV. So reliable is this means of preventing pond water from turning green that most models are sold with clear water guarantees.

The success of UV’s also lies with their simplicity of installation and use. Placed between a pump and a filter, a UV is installed to receive a constant supply of pond water. In larger systems, a separate smaller pump can be used to circulate water through a UV. Unlike other methods of controlling algae, it is impossible to overdose with UV and water treated with UV is colourless, odourless and even safe for human consumption!

To be able to choose the correct size of UV, the volume of a pond is required, the power being described in watts. The performance between manufacturers offering UV’s of similar wattage will differ as design of UV clarifiers has a significant affect on performance. Just as it is wise to choose an oversized filter if possible, similar advice is recommended when choosing a UV.


A pro-active method of preventing a virus (and bacteria) from causing disease in the future. The process of vaccination involves exposing koi to a non-lethal variety of a virus that will cause an immune response without affecting the health of a koi. This primes that koi’s immune system with the antibodies to respond rapidly to a similar viral infection in the future.

Vaccines are most effectively administered by injection, although vaccine dips have been developed where fish take up the vaccine through the gills. Vaccinations for fish are still being pioneered, where research emphasis is towards fish and viruses of commercial significance, which means food fish such as salmon and trout rather than ornamental fish. A vaccine against the bacteria causing ulcers or the latest virulent herpes-like koi virus would prove popular in the trade.


Typically made from waterproofed blue membrane, koi vats are the means by which koi are held and viewed at koi shows. 2-3 metres in diameter, these collapsible containers are put up in minutes by a clever assembly involving vertical pipes to act as supporting ribs around the perimeter, held circular by a ring of semi-rigid pipe around the circumference. The vat’s versatility means that this koi showing facility, capable of holding hundreds of gallons can fold up into the boot of a car. They are also ideal for housing koi temporarily during pond repairs and are usually available for hire from your local koi society.

Ventral fin

The third single fin on a koi after the tail dorsal fin. It is rarely seen when viewing koi as it is located behind the vent, between the koi’s abdomen and the tail. It’s role is to give stability to a koi, acting with the dorsal fin to prevent the koi from rolling side to side.


A venturi offers a means of injecting air into a pumped water return. Popular where a multi-chamber filter sits alongside a pond, a venturi is a tube that takes water, adapted with a constriction and an air intake pipe protruding above the water’s surface. Significant volumes of air can be drawn into the water flow and injected into the pond water when using a venturi.

Venturi’s can often aerate so vigorously that they disturb the water surface to the extent that they prevented unhindered viewing of koi. They also look quite impressive but tend only to aerate the upper layers of pond water.


A virus is a tiny bundle on DNA. Their minute size means that they cannot even be seen with a light microscope. Nevertheless, their influence on koi health can be quite dramatic. Viruses are parasitic in their behaviour as they require the use of a living cell to be able to divide and produce more viruses. Indeed, viruses in effect hijack host cells and make them reproduce viral DNA rather than the host cells DNA.

Viral diseases cannot be treated, but fish rely on their own immune system to defend themselves against viral attack. It is often the fish’s method of defending itself from a virus that produces noticeable symptoms.

Perhaps the most common viral complaint with koi and other pond fish is carp-pox. Although not life-threatening this herpes-type virus causes koi to produce growths on the skin. It is more common in spring, when temperatures begin to rise and the virus steals a march on the fish’s immune system. Affected fish soon clear themselves of visible signs of infection once temperatures have risen.

More recently, a new herpes-type virus that is extremely virulent, causing large mortalities in newly imported koi has been the cause of some heated debate in the industry. As this virus cannot be treated, the emphasis is on keeping koi stress to a minimum to prevent them from succumbing to a novel form of deadly infection.


A conical chamber that, through the water flow characteristics generated by its shape, cause solids to drop out of suspension. Dirty water from a bottom drain (not pumped) feeds into a vortex midway down, through an inlet that causes the water to spin around the vortex. The outlet is fitted centrally through an overflow which feds the next chamber. The passage taken by the spinning water, up and over the overflow causes more solid materials to drop out.

Subsequent vortex chambers can be installed ‘in series’, with later chambers also containing biological filter media. In this way an entire pond filter system can be based upon the vortex unit.

The settled solids can be discharged quite simply at the pull of a gate-valve for a few seconds, and new water added by a ball-valve assembly.

Ponds of varying volumes require a vortex of different capacities, relative to the size of the pond.

Kill blanketweed and string algae.