More fish are killed by poor care and overdosing/mixing medications than by disease. Always make sure you know what you are treating and the correct medication at the correct dosage is given. I do not believe in annual doses of chemicals as a preventative treatment for parasites as they will grow immune to the treatments over time.

A very popular pond treatment. can be effective against gill and body flukes.
Chloramine T will lower bacterial levels in ponds and can be used as an antibacterial dip

Antibiotics are available only on perscription from a vet who will need to examine the infected fish before any treatment is given. The best way to administer antibiotics is by injection.

Antibiotics are chemical substances produced by microorganisms that either destroy (bactericidal) or inhibit the growth of other microorganisms (bacteriostatic). Antibiotics can be either broad spectrum, which means that they are active against a wide range of microorganisms - both good and bad. Narrow spectrum drugs target a specific group of microorganisms and are able to interfere with a metabolic process specific to those organisms. In general antibiotics work by;

Preventing the synthesis of bacterial cell-wall components - typically penicillin

Damaging the bacterial cytoplasmic membrane

Interfering with protein or nucleic acid synthesis - typically tetracyclines


However, the microorganisms under attack don’t take all this sitting down. Many are able to produce enzymes that neutralise antimicrobial compounds. Others can acquire resistance to weak, short-term or repeated exposure to antibiotics. For example there are now high levels of resistance to oxolinic acid and oxytetracycline because of overuse and misuse.

Malachite Green and Formalin are one of the main cornerstones of fish disease treatments having been used for many years against a range of parasites. They can be used together or separately as anti-parasite treatments against as Gyrodactylus (skin flukes), Dactylogyrus (gill flukes), Ichthyobodo (Costia), Trichodina , Chilodonella and Ichthyophthirius (white spot).

The dynamic duo of MG&F is the treatment of choice against white spot (Ich). The relatively long active life of this treatment, together with the low filter toxicity makes it ideal for tackling the complex life cycle of this protozoan parasite. It also has a high success rate with Costia and Chilodonella and would, in most circumstances be my first choice. I have found a mixed result with Trichodina and flukes sometimes a total success, but in other cases not so good. It seems likely that the stronger dosage is needed against these two parasites and a follow up examination to assess the success of the treatment.

Malachite green has powerful anti-fungal properties and is used against Saprolegnia (fungus) either when present on fish or to as a prophylactic treatment to protect fish-eggs from infection. It is important to realise that in most cases, fungal infections of fish are secondary infections and unless the primary infection - quite often bacterial - is resolved then anti-fungal treatments such as malachite are unlikely to be successful.

Malachite green and formalin are toxic poisons

Malachite green acts as a respiratory poison, damaging the cell’s ability produce energy to drive vital metabolic processes. Formalin is a powerful disinfectant used to kill microorganisms or as a preservative for biological specimens. It works by reacting with cell proteins and nucleic acids - altering both structure and function. These chemicals can be used separately but are usually used together because they exert a synergistic effect; that is, together they give a greater effect than the sum of their separate individual capabilities.

They exert a mild anti-bacterial effect and in most circumstances will not destroy biological filtration bacteria, although they may ‘knock the filter back’ for a short while. However, it is still advisable to turn off or by-pass the filter system for a few hours if possible and as with any treatment always turn off any UV lamps. When using to treat against Ichthyophthirius (white spot or Ich) then it is important to keep the system running in order to destroy all stages of the parasite’s life cycle.

During prolonged treatments, especially when the filter has not been isolated, it is important to test for ammonia and nitrite on a daily basis in case the filter has been affected.

Using malachite green and formalin

As you might expect both of these chemicals are affected by variations in water chemistry. Both can be ‘bound out’ of the system by high dissolved and particulate organics such as fish waste, detritus and algae. Formalin is more toxic in soft, acidic water and also removes oxygen from water so vigorous aeration most be applied. Each 5mg of formalin removes 1 mg of dissolved oxygen from the water. Malachite green is also more toxic at low pH as well as high temperatures.

Malachite green has two forms depending on pH. The initial strong green coloured for prevails at low pH (acidic), while in alkaline water it is converted to a colourless carbinol form. So in alkaline water it may seem that it has disappeared, but it is still present, but invisible!

Potassium permanganate is a useful fish disease treatment; acting against a range of protozoan parasites including Trichodina, Costia and Chilodonella, as well as monogenean flukes Gyrodactylus (skin flukes) and Dactylogyrus (gill flukes). In addition to being an effective anti-parasite treatment, potassium permanganate can also assist with bacterial gill disease and bacterial disease such as skin ulcers. It will also oxidise dissolved organic matter, reducing the biological oxygen demand and improving water quality and clarity. All sounds too good to be true!

With regard to ornamental fishponds, it is mainly used in koi ponds because of dosing difficulties in heavily planted ponds.

Effective against Trichodina

My own experience is that in most cases it is very effective against Trichodina, clearing the problem quickly. Against this particular parasite it would, in most circumstances, be my first choice. I have had mixed results against flukes and Costia, but this is possibly due to the excess mucus often produced by these parasites providing protection.

A powerful oxidising agent.

As with many disease treatments, potassium permanganate is not really a medicine. It is a caustic alkali that, at high doses, can cause serious corrosion of delicate tissues such as skin and gill. In ponds it works as an oxidising agent in a similar, although milder, fashion, to household bleach! It is a very reactive chemical, reacting with organic material.

Salt, a good general disease treatment.

Salt has long been the fish-keeper's treatment standby. Salt works well against many protozoan parasites such as Costia, Trichodina and Chilodonella as well as flukes and other ectoparasites. It can also assist osmoregulation problems caused by bacterial ulcers; help clear congested gills as well as supporting fish suffering from stress. Because it works in a different way to most disease treatment, it is safer than many pond treatments and will not adversely affect biological filtration in pond filters. It is generally used at fairly high rates in short-term baths or dips, but can be used as a long-term supportive treatment in ponds.

Kills parasites

Its action is based on altering the osmotic gradient between the parasite or fish and the surrounding water. Osmosis, as you will recall, is the movement of water from an area of high concentration (where the water contains less dissolved substances and therefore more free water molecules) to an area of less concentration (where the water contains more dissolved substances and fewer free water molecule).

This is a difficult concept to grasp and explain, but it means that in a freshwater pond or tank there is a continuous movement of water from the pond water into the parasite's body fluids which contain dissolved molecules such as proteins, salts, ions etc. Most aquatic organisms such as fish and parasites control this continuous influx via osmoregulation – literally regulating osmosis. (see water quality articles for more info). The details are not important as long as we understand that osmosis takes place and that in freshwater, water is being continuously drawn into fish and other organisms.

When topical treatments are used it is important to thoroughly debride the affected area and remove all necrotic tissue and debris from the lesion. This removes a source of continuing infection and helps the healing process to start. Where there are large areas of ulceration, topical treatments may assist osmotic stress by sealing the area against water ingress, albeit for only a short time.

Thoroughly clean the lesion

The fish really needs to be sedated for this procedure (see anaesthetics pages). The lesion is thoroughly cleaned using an antiseptic solution such as povodine-iodine or similar using a cotton bud or small swab of cotton wool. If there are any infected scales these should be removed, otherwise the scale pocket will act as a continuing reservoir of infection. In the case of fin rot it may be necessary to trim the infected area back to healthy tissue using a pair of sharp, sterile scissors.

When the lesion is clean it should be dried with a tissue and covered with waterproof cream or ointment. There are proprietary products available. I tend to use Orabase from Boots the chemist. The secret with this product is to only use a small amount but spread it thinly using a wetted finger. The thinner it is spread, the longer it stays in place.

Avoid repeated debridement

Topical treatments can be successful on their own if the infection is not too severe. They can also be used in conjunction with bath treatments or antibiotics injections for enhanced effects. The procedure may need to be repeated again a day or two later, but repeated debridement of the lesion should be avoided as this may “wipe” away new skin growth. Fish integument heals by a thin layer of skin cells migrating across the lesion from the adjacent healthy tissue.

If, after the initial treatment it is felt that the fish would benefit from further treatment to the affected area, the lesion should be lightly swabbed with an antiseptic solution and the top dressing applied very carefully

Assuming that the lesion was thoroughly cleaned initially one would expect to see some signs of improvement by way of reduced inflammation or signs of new skin growth within 5 –7 days. If after this time the lesion shows no sign of healing a course of appropriate antibiotics may well be needed.