How do pond treatments work – and how to use them.

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.

Chemotherapy in Fish

Chemotherapy in aquatic environments is quite different from treatment in terrestrial animals. Instead of treating the individual animal directly, the water itself is usually treated. The chemical dose is calculated according to the volume of the aquarium or pond rather than the size or number of fish present.

For long-term bath treatments, it is therefore essential to know the precise water volume. This approach offers clear advantages: fish can be treated without handling, and large numbers of fish can be medicated simultaneously. However, the cost of treatment increases with water volume. Treating twenty fish in a large pond will inevitably cost more than treating twenty fish in a small aquarium.

In certain cases — particularly with large specimen koi suffering from specific bacterial infections — individual antibiotic injections may be more appropriate. In these situations, dosage is calculated according to the fish’s weight, and treatment costs are directly related to the size of the fish.

Understanding Chemotherapy

The term “chemotherapy” often brings to mind cancer treatment and severe side effects. Those side effects occur because the treatment is toxic not only to the disease but also to the host.

The same principle applies in fish health management: the aim is to use a dose strong enough to eliminate the pathogen but not so strong that it harms the fish.

Under-dosing is particularly dangerous. It stresses the fish without fully eliminating the pathogen and may allow surviving organisms to develop resistance. This is already seen in some antibiotic-resistant bacteria and parasites that have developed resistance to certain treatments due to inappropriate use.

In fish health, pathogens fall into four main categories: viruses, bacteria, fungi and parasites.

Viral Diseases

Viruses cause diseases such as carp pox and serious notifiable conditions like Spring Viraemia of Carp (SVC).

Viral diseases cannot be treated with medication because of the way viruses operate inside host cells. Once infected, recovery depends entirely on the fish’s immune response.

Notifiable diseases are legally regulated conditions that must be reported to the relevant authority. Movement of infected fish is strictly controlled to prevent spread. Such regulations have resulted in the closure of many fish farms following outbreaks.

Vaccination is possible for certain viral diseases, particularly in commercial fish farming. Fish are exposed to a weakened form of the virus to stimulate antibody production. Once vaccinated, the fish develops immunity to future infection. Vaccination can be administered via injection or immersion (dip or bath methods).

Bacterial Diseases

Bacteria commonly cause external problems such as fin rot, gill rot, ulcers and so-called “mouth fungus” (which is actually bacterial in origin).

External bacterial infections are often treated by dosing the water with an appropriate antibacterial treatment. Internal bacterial infections — which may present as haemorrhaging or dropsy — are better treated with antibiotic injections.

Antibiotics are prescription-only medicines and must be obtained from a qualified veterinarian. Injections are generally suitable only for larger fish. In some cases, medicated food may also be prescribed.

Fungal Infections

Fungal infections typically appear as cotton wool-like growths on wounds or damaged tissue.

Fungus can be challenging to treat, particularly in advanced cases. The visible fluffy growth is only the surface manifestation; beneath it, fungal filaments may penetrate deeply into living tissue.

Unlike many bacterial and parasitic diseases, fungal infections are not contagious. Healthy, intact fish are unlikely to become infected unless they have damaged tissue.

Effective fish chemotherapy requires accurate diagnosis, correct dosing, and responsible use of medications. Treating unnecessarily or incorrectly can cause more harm than good.

4. 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

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.

Acriflavine

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.

Table 1. Effect of Medications on Freshwater Biofiltration

Medication Concentration          (mg/l)       % Filter Inhibition
Oxytetracycline (Antibiotic)          50            0
Erythromycin (Antibiotic)             10            100
Formalin                                           6 – 10       0 – 27
Malachite Green                              0.1           0
Formalin + Malachite                    10 + 0.1  0
Methylene Blue                               5               100
Potassium Permanganate             1 – 4         0 – 86

Antibiotics

The term antibiotic literally means “against life.” Antibiotics are substances originally derived from fungi or bacteria that inhibit or kill other bacteria. Although many were first discovered in nature, most modern antibiotics are now manufactured synthetically while retaining the same antibacterial properties.

In aquatic medicine, antibiotics are used to treat bacterial infections such as ulcers, septicaemia and other systemic conditions. Because of their importance and potential for misuse, antibiotics are controlled medicines and are only available by veterinary prescription.

This restriction exists to prevent overuse and inappropriate application, both of which contribute to bacterial resistance. When antibiotics are used unnecessarily or incorrectly — for example, at too low a dose or for too short a duration — some bacteria survive and develop resistance. These resistant strains can then multiply, making future infections much harder to treat.

Antibiotic resistance is already a significant issue in fish health. Drugs such as oxolinic acid and oxytetracycline have become ineffective against many bacterial strains. This has been particularly problematic in imported pond fish, where ulcers sometimes fail to respond to treatment because the causative bacteria are resistant.

Responsible antibiotic use is therefore essential. Accurate diagnosis, correct dosing and full completion of prescribed treatment courses are critical to preserving the effectiveness of these important medicines.

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

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.

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

2. 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.