The Importance of Using Malachite Green in Aquariums

Fish swimming in aquarium
Hidehiro Kigawa / Getty Images

Name: Malachite Green

Other Names: aniline green, benzaldehyde green, china green

Action: Triarulmaethane dye, antibacterial, antifungal, anti-parasitic.

History & Uses: First made in 1877, Malachite Green has been used primarily as a dye for fabrics, leather, and paper. It is also used as a biological stain. In aquaculture, it is often used as an anti-fungal for eggs in hatcheries, in spite of growing concern regarding potential health hazards.


Malachite Green is often combined with formaldehyde (aka formalin) and is somewhat similar to Methylene Blue in terms of uses. However, it is a harsher chemical and must be used with caution.

It is most effective against external parasites, particularly when combined with formaldehyde. Malachite Green is also quite effective in controlling fungus on fish eggs. Disorders treated by Malachite Green include the following:

  • Egg Fungus - Serves as a safe disinfectant for fish eggs.
  • Ichthyophthirius multi files - Small white spots resembling sand, fish scratches against objects.
  • Oödinium pillars - Also known as Velvet, this is actually caused by a parasite.
  • Saprolegnia - A freshwater mold that often attacks fish eggs.

Manufacturers Directions for Use: Use 1 teaspoon (approximately 5 ml) per 10 gallons of water for most treatments. This produces a concentration of 0.05 ppm.

Prevention or treatment of fungus on fish eggs:

Method 1 (Short Term Bath)

  • Change the carbon in the outside filter or add an outside filter with fresh carbon to the hatchery tank. Use a high-quality liquid phase activated carbon.
  • Start the filter.
  • Add 10 teaspoons of 0.038% Kordon Malachite Green per gallon of water. This produces 5 ppm activity of the drug.
  • Allow the filter to run and allow it to remove the Malachite Green from the water for one hour. Note: If any color remains in the water after 1 hour, perform a partial water change of at least 25%, change filter carbon and allow it to remove any additional Malachite Green.
  • Repeat treatment once a day prior to the hatching of the eggs. Discontinue after the eggs hatch.

Method 2 (Dip)

  • To a clean, non-metallic container, add 1 gallon of aged hatchery water, or water treated with NovAqua (DFS# 21207, 21208).
  • Add 5 Fl. oz. of 0.038% Kordon Malachite Green (approximately 149 ml). This produces a concentration of 1500 ppm.
  • Agitate the solution with an airstone and adjust for a moderately strong flow of air.
  • Collect eggs in a net or grasp the object on which eggs are attached and dip them for not more than 10 seconds. Immediately replace the eggs into the original hatchery tank.


This is a one-time treatment only. Do not repeat. The dip solution may be kept for future use. Keep container tightly capped when not in use.

Treatment for Fungus on Fishes

  • Transfer fishes to a separate quarantine tank, if possible.
  • Remove carbon, clean outside filter, and return to use with clean mechanical filter media.
  • Calculate the actual volume of water to be treated, taking into consideration the displacement of water by sand and rock.
  • Add 1 teaspoon of Malachite Green per 10 gallons of water to the tank. This will produce a concentration of 0.05 ppm.
  • Make a partial water change of at least 25% every 24 hours and re-treat. Superficial fungal infections will respond to a single treatment, while more advanced, deep-seated mycosis will require additional prolonged therapy. The disappearance of the strands (fungal hyphae) from the affected areas signals a successful treatment.
  • Upon completion of the treatment, return fishes to the original aquarium.

Kordon Rid-Ich Plus  

Manufacturers Directions for Use:

Dosage: Use 1 teaspoon (approximately 5 ml) per 10 gallons of water. This produces a concentration of 15 ppm formalin and 0.05 ppm of malachite green.

Treatment of Freshwater and Saltwater External Parasites: The following procedure is applicable when treating the diseased fishes in their original aquarium or pond:

  1. Remove any invertebrates you wish to save.
  2. Remove carbon and clean the filter. Replace with clean mechanical filter media and return to use without the carbon.
  3. Perform a partial water change of at least 25%. Repeat water changes before each re-treatment with Rid·Ich+.
  4. Calculate the actual volume of water to be treated, taking into consideration the displacement of water by gravel, rock, and ornaments. (To calculate the aquarium's capacity measure its length, height, and width in inches, multiply these dimensions together and divide the result by 232. Your answer will be the amount of water in gallons.)
  5. Add 1 teaspoon of Rid·Ich+ per 10 gallons of water. This produces a concentration of 15 ppm of formalin and 0.05 ppm of malachite green.

Treatments may be repeated as often as every 6 hours (depending upon the severity of the particular disease and the tolerance of the diseased fishes). Treatment should be repeated no less than every 24 hours (see Step #3) and continued for at least 3 days beyond the disappearance of all signs of disease. After treatment, make a 40% water change.

Para Guard 

Manufacturers Directions for Use: In the aquarium, use 5 mL (1 capful) to every 40 L (10 gallon). Repeat daily as required as long as fish show no stress. For 1-hour dips, use 3 mL per 4 L (1 gallon). Dips may be extended if fish show no evidence of stress. In case of eye exposure, promptly & thoroughly wash eyes with water & seek medical attention.

Quick Cure 

Manufacturer's Directions for Use: Shake well before using. Remove and discard all carbon from the filter, but DO NOT discontinue filtration. Typically cures ich in 24 hours, treatment can be repeated in 24-48 hours, if necessary. If fish experience stress, change 50% of the water. Sometimes treatment will take 10 days to fully irradiate infestations.

Carbon or other similar chemical filter media can be replaced after treatment is finished or added to the filter temporarily for 2 hours prior to each subsequent treatment then removed again to lower toxicity (correct mineralization and a pH over 7 is also important. Quick Cure will temporarily change the water color to blue.

Dosing Chart:

  • 1 ml treats 18 gallons (68 liters)
  • 1 teaspoon treats 90 gallons (340 liters)
  • 1 capful treats 250 gallons (950 liters)
  • 1 fluid oz. Treats 500 gallons (1900 liters)

For Tetras or other delicate fish such as loaches, treat half dose.