M: Pond Keepers A to Z – Malachite Green To Mucus

Malachite Green

Malachite is an intense dark green dye that is widely used in the treatment of many koi ailments. It has even historically been used as an antiseptic in the treatment of wounds in humans. Different grades are available but the less toxic zinc-free malachite green is used in the treatment of koi and other fish to treat fungus and microscopic external parasites such as Chilodinella, Costia and White Spot.

It is so effective that most proprietary medications on the market are dark green in colour because of the malachite green content. Due to its broad action, malachite is often the foundation to many aquatic medications. Malachite is toxic to humans as well as fish and is a cancer-causing substance. Care must be taken when using it especially if in powder form as it is extremely concentrated in this form. Fish unavoidably absorb malachite through their gills and being a cumulative toxin it is stored in fish flesh. Its use in trout farming has recently been brought into question, as there is evidence that fish reaching the market contain traces of malachite green.

Malachite works by permeating cell membranes of parasites and fungi where it interferes with respiratory and metabolic processes within the cells. Consequently, treated pathogens are unable to generate energy within their cells, eventually dying. Although it is toxic to fish, they remain unharmed, as the doses that are effective against parasites do not adversely affect koi health.

If koi are overdosed with malachite, their cells suffer the same effects but on a massive scale, potentially killing the fish. There is no antidote for malachite over dose. Malachite green is most widely used in a solution of formalin (a 40% solution of formaldehyde gas) as an effective treatment for fungal, bacterial and parasitic problems. Sometimes referred to as Leteux-Meyer Mixture, the effects of formalin and malachite mixture is greater than the sum of each if used individually.

Mechanical filter

Filtration lies at the heart of a koi pond and is the key to excellent water quality, which in turn leads to enhanced koi health and growth. A filter should be designed as an integral part of the pond, with allowances made during the ponds design and construction for the space and expense required by a filter. The two essential areas of a filter in a koi pond can be divided by their function, namely mechanical and biological. (Sterilisation and chemical filtration are largely regarded as optional).

The biological filter acts to house beneficial bacteria that will break down toxic waste products excreted by fish into less harmful by-products. A biological filter acts on the ‘invisible’ and soluble waste. Prior to water from a pond entering a biological filter, it must pass through a mechanical phase that will enhance the biological function and improve water clarity.

A mechanical filter’s function is to remove all of the suspended particulate matter from the water. The limiting factor of most filter systems is their ability to adequately remove solids that will go on and cloud the water. If a filter problem occurs, then it is likely to be related to inadequate mechanical filtration rather than biological filtration. The mechanical chambers or areas of a filter are those that are cleaned and maintained most regularly and should be as large as possible to reduce the frequency of cleaning. Since the introduction of UVCs the role of a mechanical filter has been greatly increased, as it has to handle excessive quantities of a sticky algal clumps.

Mechanical filters can work in a number of ways:-

Settlement. The velocity of water is reduced which leads to suspended matter dropping out and settling to the bottom.

Vortex. A physical interaction of a vortex causes suspended matter to drop out of suspension.

Entrapment. An array of media, from brushes to foam and even gravel can be used to trap and remove particulate matter. The efficiency of entrapment increases with the age of a filter as bacteria colonise the chamber coating the media in a ‘sticky’ layer, trapping and collecting particulate matter very effectively.


An all embracing term covering the range of chemicals and other therapeutic agents available for the treatment of almost any koi health problem. They can be obtained (and often make an impressive display) from any aquatic store. Some specific treatments such as antibiotics can only be obtained via prescription from a Vet.

The majority of medications available concern the treatment of external (and also largely visible) diseases and symptoms and are administered indirectly by dosing the pond water. This is why it is essential for pond owners to accurately know the volume of their pond.

Perhaps unlike any other ‘pet’, the cost of treating any number of koi will be the same, irrespective of the number of fish in the pond.

Besides these long-term ‘baths’ where the medication is added to the pond, medications can be applied in a number of other ways.

- Dips. Where individually affected fish are removed and given a short-term but relatively concentrated bath. This saves having to treat the whole pond unnecessarily, as long as the offending fish can be easily caught!

- Topical Treatment. Again, for individually affected fish, a topical treatment is where the medication is applied directly onto the affected area. The koi will more than likely have to be anaesthetised, this treatment can be very effective against ulcers, and may also involve the removal of a number of damaged or degraded scales.

- Oral Medication. Where medicated feed (containing antibiotics) is fed to koi in an attempt to treat against symptoms such as septicaemia (blood poisoning). As these are antibiotic-based preparations, they are only available via prescription.

- Injection. This method is only carried out on larger fish that are demonstrating wholesale bacterial infections or the combination of other secondary symptoms. An injection is a very valuable method of administering an accurate dose of a specific medication to an individual fish. It is however, a skilled method and requires great attention to detail for the ultimate care of the fish.


An essential aid for the serious Koi keeper, that empowers the user to positively identify the pathogens causing an external disease. Effective microscopes are available from just over 100 right through to over 1,000. Fresh samples of mucus for viewing under a microscope can be collected from suspect koi using a glass microscope slide, pulling it gently along the dorsal flank of a koi in a backwards direction. The mucus can then be ‘squashed’ under a wet cover slip, and viewed under low power magnification.

The typical ‘light microscope’ will have a light source beneath the slide shining up and through the two lenses of the microscope. The two lenses are the eye piece (typically x10 magnification) and the objective lens, (closest to the slide and ranging from x2.5 to x100). Therefore the compound magnification of the microscope will range from 25-1000x.

The mucus sample should be viewed at the lowest magnification first to make focussing easy, and then viewed if necessary under progressively more powerful objective lens. Amongst the random textured mass of a mucus sample, offending parasites can easily be viewed as those well-formed, regularly shaped and moving objects. These can then be easily identified using an appropriate guide on disease organisms.

A positive identification allows the accurate diagnosis and administering of medication.

Minerals Water

from whatever source, will contain a mix of dissolved inorganic materials. Minerals are an essential requirement for the health, well being, and in some instances, colouration of koi.

Hard water contains relatively high levels of dissolved minerals, such as calcium and magnesium, which can interact with other dissolved substances to form other compounds. (This is why, after treating a pond with malachite green, that the dye will remain visible for longer in ponds containing softer water).

Fish have an advantage over ourselves, in that they can gain their mineral requirements from both their diet as well as their environment.

Koi prefer a hard and mineral-rich water. Such waters will have a desirable alkaline pH that will also be quite stable. Over time, the profile of specific minerals can become diminished, as they are selectively taken up and utilised by koi and their pond life and the desirable mineral content can be improved by using certain pond additives.

Such a typical additive is clay, specifically montmorillonite. Although the clay itself is insoluble and will be removed by the filter, it brings with it a rich blend of minerals which may be released into the pond water. Reports of increased vigour and colour are regularly reported by koi keepers a week or so after adding the refreshing tonic of montmorillonite to a previously minerally deficient and ‘tired’ ponds.

Mirror Scale

Compared with the standard smaller scale as found on the majority of fully scaled koi, the mirror scale is approximately 6-8 times the size. Found on Doitsu varieties of koi, (Doitsu is ‘Japanese’ for German-Deutsche), the scale type and pattern is reported to have been inherited from German/European carp stock.

Mirror scaled, or Doitsu Koi, can have a wide variation in distribution and number of mirror scales, however, only those exhibiting mirror scales along the dorsal and lateral lines and retained after culling as these are regarded as showing the desirable mirror scale pattern.

Monogenetic Fluke

Generally translated as ‘one beginning’, meaning that its life cycle involves koi, and not other organisms.

A typical monogenetic fluke is the skin fluke, Gyrodactylus. This produces live young that are released by their parents that will infect a host koi after a brief period in the water.

Monogenetic flukes are ideal candidates to be viewed on a mucus scrape under a microscope. They are identified by their two painfully looking hooks that are used to attach onto the skin of koi. They can regularly be seen with live juveniles moving and twitching within the parent’s translucent body, ready to be released. Symptoms of infected koi are the typical scratching and flashing behaviour, as koi try to rid themselves of these external pests. They are best treated using organophosphorous compounds, but formalin based medications can also prove effective.

Mouth Fungus

Just as pond fish may suffer from mouth fungus, mouth fungus suffers from being poorly described. Its common name describes the appearance of this disease rather than accurately describing the decease organism that causes it. Unusually, the cotton wool-like growths around the mouth (and occasionally the fins and lost scales), is not caused by a fungus (as we may suspect), but actually a specific type of bacteria.

Unlike fungus, mouth fungus (caused by Flexibacter bacteria) is contagious and is likely to invade areas of a koi’s body that have lost their first line of defence; – perhaps the mucus and some skin may have to be eroded away. As koi and other pond fish are constantly foraging and grazing in sediment or pond surfaces then mouth fungus infection is always a likelihood.

Fortunately, on a practical koi keeping level, mixing mouth fungus with a normal fungal infection does not lead to problems when treating affected fish. Broad spectrum anti-bacterial treatments based on formalin and malachite solutions will be effective in most cases. Depending on the severity of the site of the infection, repeated pond treatment should suffice with topical treatments only being necessary in extreme cases.


Mucus is the multifunctional ‘finish’ to a koi. It is a very simple protein, produced by cells in the skin of koi which acts as a first line of defence in the protection against disease. It’s slimy nature also affords koi (and all fish) a means of improving its hydrodynamics, as it reduces the friction between the water and the fish as it moves through the water.

Unfortunately, the mucus that koi produce can also prove irresistible as a source of nourishment for external parasites, which hop aboard and feed on this protein diet.

In extreme cases, the colours of koi can appear to fade as they are masked under an unusually deep covering of mucus, produced in response to the irritation caused by the pathogens. Excessive sliminess of skin or fading colours can be useful indicators of an external parasitic infection.

Kill blanketweed and string algae.