Pond fish koi medications. How do they work?

Ever since Adam and Eve sinned in the Garden of Eden, man has been struggling with the problems of an imperfect world such as weeds, death and disease. This is very true for the aquarist who over the years has been forced to experiment with different chemicals and compounds in the quest to find the ‘magic bullet’ that will keep his fish free of disease.

Aquarium and pond treatments are big business and it is evident how much fishkeepers are cursed with disease when viewing the range of bottles, boxes and powders stocked by aquatic dealers to treat fish.

Once opened, many of the different branded treatments look very similar as they share very similar formulations, tried and tested over time. There is very little financial incentive for pharmaceutical companies to research new aquatic medications when confronted with extensive research costs and the limited market compared with the incentives for finding new drugs for the human medicine.

Many of the chemicals used in the hobby today have been transferred from fish farming and other disciplines, including human medicine.

As science becomes progressively applied to the hobby and in this age of information overload, more questions are being asked of what fish are subjected to when treated for disease. Are the treatments environmentally friendly? Are there alternatives? Why do we use them? How do they work?

All medications are used for their toxic effects on the target disease organism, whether bacteria, fungi or parasites.

Unfortunately, most medications are also toxic to fish as well as filter bacteria and aquatic plants.

The approach to chemotherapy in the aquatic environment is quite unique in that the water is usually treated instead of the animal. The chemical dose is determined by the volume of the aquarium or pond rather than the size or number of fish. In this way, when using a long-term bath treatment it is necessary to know the precise volume of the tank or pond rather than the size or weight of the diseased specimens.

This has many obvious benefits in that the fish can be treated without being handled and potentially thousands of fish can be treated in a single action. However, the cost of treating 20 fish in a pond compared with 20 fish in an aquarium would unavoidably be more expensive.

Larger specimen fish such as koi and other large pond fish suffering from certain bacterial conditions are sometimes better treated individually with antibiotic injections. In contrast, in this situation it is essential to know the size of the fish and the cost of the treatment is directly related to the weight of fish treated.

Chemotherapy (chemical treatment) immediately conjures up thoughts and images of cancer treatment and the appalling associated side effects such as loss of weight and hair. Such graphic side effects are due to the treatment also being toxic to the host. The key factor is treating with a dose that is sufficiently concentrated to kill the disease but not the host. The same is true in fish chemotherapy.

It is better not to treat at all rather than under-dose as under-dosing stresses the fish and does not eradicate the target pathogen, perhaps even enabling future generations of that disease to become resistant to treatment. This is already the case with some bacteria that are resistant to antibiotics and some parasites being resistant to insecticides where the application of the medication has been inappropriate.

There are 4 categories of pathogen (disease-causing organisms): viruses, bacteria, fungi and parasites.

1. Very briefly, viruses cause diseases such as carp pox and are responsible for a number of notifiable diseases such as SVC in carp. Viral diseases cannot be treated because of their mode of action and this is why most of the notifiable diseases in the UK are viral. Once a fish has contracted the disease, it is untreatable and down to the fish’s own immune response to attack the virus. (TEXT BOX: Notifiable disease is a disease listed by MAFF whereby any occurrence of such a disease must be reported to MAFF who control the movement of these fish and thus control the spread of the untreatable viral disease. Notifiable diseases are responsible for closing down many fish farms as a result of the strict practices that infected sites have to carry out.)

Fish, just like humans, can be vaccinated against certain viral diseases where they are inoculated with a weakened form of the virus that stimulates the body to produce anti-bodies against that virus. In future infections by that virus the body is at an advantage in that it already possesses the antibodies to attack the virus. The fish is then immune to that virus. Fish vaccination is more common in fish farming and can be carried out using a dip or bath.

Bacteria are responsible for causing external complaints such as fin rot, gill rot, ulcers and ‘mouth fungus’ and are usually treated by dosing the water with a chemical treatment. Internal bacterial complaints which may cause haemorrhaging and possibly dropsy are better targeted using an injection of antibiotics. Antibiotics are only available under prescription from a vet and injections are only really suitable for larger fish. Vets can also prescribe antibiotic food.

Fungus is responsible for the cotton wool-like growths found on wounds or abrasions. Fungus is often quite difficult to treat, especially in advanced cases, as the protruding fungus growth is the tip of the iceberg in that out of sight, the fungus is penetrating deep into the living fish tissue. Unlike the other categories of disease, fungus is not contagious and affected fish will not infect healthy and intact fish.

Parasites can attack the exterior and interior of fish and range in size from being microscopic (Chilodinella, Trichodina) to those visible by the naked eye (Fish Louse, Anchor Worm, White Spot) up to those several centimetres long such as leeches and tapeworms.

Parasites on wild fish remain in a finely balanced relationship where their level of infection does not cause the death of the host. It is in the parasite’s interests to keep its host alive. In captivity, however, fish and parasite relationships can become unbalanced causing the death of the host. Treatments for parasites vary according to the location of the parasite (internally/externally), the size of the parasite and its lifecycle.

What treatments are used?

It is no coincidence that many proprietary aquarium and pond medications are very similar in colour and appearance. Most of them follow very similar formulae with differences occurring in the refinement of chemicals used, chemical concentrations and minor adjustments to the base formula.

Other chemicals are used on their own and for toxic implications cannot be mixed with other active ingredients.

Commonly used chemicals include malachite green, formalin, acriflavine, potassium permanganate, methylene blue, antibiotics and salt. An aquarist at one time or other will have used at least one of the above to treat fish so it is pertinent to know what effect these chemicals have on the disease, fish and living aquarium.

How do medications work?

Malachite Green

Malachite is a strong dark green dye that has even been used as an antiseptic in wounds in humans. Different grades are available but the less toxic zinc-free malachite green is used in aquatics to treat fungus and microscopic external parasites such as Chilodinella, Costia and White Spot.

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.

A replacement for malachite that is less risky to humans is being researched but as malachite is so effective and readily available, the financial incentive to research and develop an alternative is very small when considering the small size of the aquatic market. Such a change will only occur through legislation banning the use of malachite in certain circumstances.

Malachite permeates through 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.

If fish are overdosed with malachite their cells suffer the same effects on a massive scale potentially killing the fish. There is no antidote for malachite over dose.


Formalin is a solution of approximately 40% formaldehyde gas. It is a clear, colourless, pungent and highly toxic solution. Regularly used with malachite green (Leteux-Meyer mixture) as the effect of the 2 combined chemicals is greater than the sum of each if used individually. It is very effective against microscopic external parasites such as white spot, Costia and velvet disease (Oodinium).

Formalin is a universal disinfectant and works by damaging the structure of proteins that form the structure of cells and the genetic information within the cells themselves. Technically described as a protein precipitator formalin denatures proteins unselectively including fish tissue so the correct dosing is critical.


The antiseptic nature of acriflavine is known from its uses in human medicine as a treatment for mouth and throat infections and for disinfecting wounds.

It is a deep yellow powder that dissolves easily in water and can be used to treat bacteria, fungi and particular parasitic infections.

It is absorbed through cell membranes where it reacts with DNA inside the cells. This disrupts the pathogen’s ability to reproduce causing an accelerated death and preventing the spread of the infestation. It is still not clear why acriflavine has this effect on DNA within cells.

Potassium Permanganate

This is a dark purple crystal effective against bacteria and external parasitic infections. It is a very toxic chemical that can quite easily kill fish if even slightly over dosed. Its mode of action can be seen by the naked eye in that it forms manganese dioxide giving surfaces a brown colouration. This forms a manganese-protein complex in contact with proteins (fish skin, mucus, parasites, bacteria) which interferes with the protein synthesis of the organism, causing death.

Its effect is greatly reduced by a high organic content in the water as the potassium permanganate reacts with the organic molecules in the water rather than the target organisms.

In the Second World War and during rations, when tights and stockings were in very short supply, to remain fashionable, ladies regularly took Potassium Permanganate baths to turn their legs brown! This is not to be recommended.

Methylene Blue

Methylene blue comes as a very dark green powder, appearing blue when dissolved in water. Considered a ‘traditional’ medication for bacterial, fungal and parasitic infections, it is used less frequently as it is highly toxic to plants and will wipe out the bacteria in a biofilter (See Table 1). It is easily absorbed through cell membranes and affects cell activity by raising oxygen consumption within cells. Its mode of action is unclear but it is thought that its action is similar to that of other dyes.

Antibiotics (literally means ‘against life’)

Antibiotics are naturally occurring chemicals produced by fungi or bacteria that have an antibacterial effect. Antibiotics are now manufactured and produced synthetically but have the same effect.

The use of antibiotics is controlled by prescription to limit their use to treating worthy cases rather than allow their widespread use to prevent disease that would increase bacterial resistance. This has already been seen with antibiotics such as oxolinic acid and oxytetracycline which are now useless against many bacteria. This has been one of the major issues recently when treating ulcers in imported pondfish where the ulcer does not respond to antibiotic treatment due to bacterial resistance.

Antibiotics can be added to water in short term baths and dips, fed orally in medicated food or given via injection. They must not be added to a tank or pond as they will wipe out any biofiltration (See Table 1).

Their mode of action is varied and ranges from the interference of cell membrane formation in developing bacteria to the inhibition of genetic apparatus within microbes, preventing cell division and the multiplication of bacteria. As bacterial infections spread rapidly it is vital to stop the bacteria from dividing and multiplying and antibiotics achieve just that, unless the bacteria are resistant to that antibiotic.


Salt (sodium chloride) can have several therapeutic effects on fish and is used quite regularly by some koi keepers.

It has effective antiseptic properties and can be used as a tonic in mild concentrations to stimulate the fish’s metabolism. It can also be added to ponds or aquaria to reduce nitrite toxicity. Used as a dip or a long-term bath, salt can be used as a treatment against external parasites such as Trichodina, Argulus and Lernaea.

In stronger concentrations, salt is believed to have a 2-fold effect on parasites.

Acts as a skin irritant increasing mucus production, throwing off skin parasites.

The high sodium ion concentration is toxic to external parasites.

If treating ulcerated fish, the addition of salt to the water also reduces the influx of water into the fish, taking pressure off its kidney functions. It is important to remember that salt is persistent in a system until it is removed with a water change.

As it can be seen, the mode of action of many fish medications is at the cellular level, attacking cell membranes or activities within the cell. Medications are not usually selective in their action but quite crude, with fish surviving by virtue of their size and complexity in relation to the susceptible pathogen. Consequently, pathogens of just 1 or a few cells in size are easier to treat than the more complex and larger parasites.

Medications are nearly always toxic to fish and humans and administered on the basis that at the recommended dose rates, they are more toxic to the pathogen that the fish. This is why medications should never be overdosed.

It is essential to acknowledge that while most disease problems are the result of a water quality problem, aquatic medications will not solve a water quality problem but only treat the disease. Therefore always try to identify and solve the cause of the problem before treating it.

Kill blanketweed and string algae.