Malachite green and formalin a good general-purpose anti-parasite treatment

Malachite green and formalin treatments

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!

More on malachite green and formalin:

  • Wear gloves and goggles when handling malachite green or formalin. Do not inhale the fumes of formalin and always handle in a well-ventilated area. Both are potentially dangerous. Malachite is a respiratory poison as well as being a suspected carcinogen.
  • Malachite green penetrates deeply into fish body tissues and may have a use against Proliferative Kidney Disease (PKD) in salmonids.
  • It is likely that malachite green will be banned in the near future
  • Malachite green is de-activated by light
  • Malachite green is reported to be toxic to tetras, catfish and loaches and small marine fish
  • Formalin should be stored in the dark and above 40C to prevent the formation of paraformaldehyde
  • Formalin is extremely irritation to gills and should not be used where there may be existing gill damage or where skin ulcers are present.

Dosage rates:

Formalin: Used against protozoan and metazoan parasites.

To prevent any misunderstandings, formaldehyde is a colourless, highly toxic gas. Formalin is a 37 – 40% aqueous solution of formaldehyde (which equals 100% formalin). It should not be used if a white precipitate of paraformaldehyde forms in the container.

Paraformaldehyde is extremely toxic to fish. Keep formalin away from light and cold. Be extremely careful when handling.

  • Bath: 0.15 to 0.25 mls per litre for up to 60 minutes.; Can be used on consecutive days for a  maximum of three treatments. Can irritate gills so it should not be used where gill disease is suspected. Aerate at all times. In most cases the lower dose should be used although the high dose may be required against Epistylis
  • Prolonged immersion: 0.015 to 0.025 mls per litre. Repeat every 3 – 4 days and do a partial water change between treatments. Maximum of three consecutive treatments. Aerate at all times. Do not use where gill disease is suspected.

Malachite greenUsed against Saprolegnia (fungus), water moulds and protozoan parasites.

A zinc-free grade must be used. This is usually mixed as a stock solution that will then keep indefinitely. The exact mixture of the stock solution varies depending on preference. The main point is that whatever concentration is used it should be easy to calculated many mgs of malachite there are per ml of solution. A popular stock solution uses 20 grams malachite per litre of distilled water. This gives 20 mg malachite per ml of stock solution. Using this stock solution.

Bath: 1-2 mg malachite per litre water for 30 – 60 minutes. Higher dose only for large fish, such as koi, in hard water.  This equates with 1 ml stock solution per 20 – 40 litres of water. Can be repeated every other day for a maximum of four treatments.
Prolonged immersion:0.1- 0.25 mg malachite per litre: Repeat every three days for a maximum of three treatments. This equates to 1ml of stock solution per 80 – 200 litres. Again the higher dose should only be used with large fish, such as koi, in hard water.
Topical treatment: The stock solution can be applied directly to a wound, particularly when fungus is present. Keep away from the fish’s eyes and gills.

Malachite and formalin mixture (Leteux-Meyer mixture) Used against protozoan and metazoan parasites.

There are several variations. Two commonly used mixtures are:

  • Strong mixture: 3.68 grams of malachite green dissolved in one litre of formalin: This is used at 0.025 mls per litre of pond water for 60 minutes bath. This stronger dose can also be used for stubborn parasites on koi in alkaline water as a prolonged immersion. This dosage equates to 0.025 ml/litre formalin and 0.1 mg. /litre of malachite green.
  • Weaker mixture: 3.3 grams malachite green dissolved in one litre of formalin: This is used at a rate of 0.015 mls per litre of pond water as a prolonged immersion for general pond use. This equates to 0.015 mls/litre formalin and 0.05 mg/litre malachite green

See the fish disease section for more details about specific parasites or diseases

Useful conversions are:

ppm = mg/litre            i.e.  5 ppm = 5 mg / litre

mg / litre  x 3.785 = mg / gall (US)           i.e  5 mg / litre = 18.9 mg / gall (US)

mg/ litre x 4.546 = mg / gall (UK)            i.e  5 mg / litre = 22.7 mg / gall (UK)

To convert imperial gallons to US gallons multiply by 1.2

Other useful figures: 

1 ounce = 28.35 grams

1% solution = 

10 ml per litre

10 gram per litre

38 gram per gall  (US) 

45 gram per gall (UK)