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A fungus (plural fungi) is a eukaryotic organism that digests its food externally and absorbs the nutrient molecules into its cells. Along with bacteria, fungi are the primary decomposers of dead organic matter in most terrestrial ecosystems.
Fungus propagate via billions of spores which travel through the air and water and settle on all surfaces waiting for a food source to appear and so the fungus can be said to be everywhere waiting for the right conditions.
Actively growing fungus (typically Saprolegnia, Achlya, Leptomitus, Pythium, and for marine Exophiala, Ichthyophonus) in an aquarium is primary a sign of low quality aquarium conditions by the owner. It appears as fluffy white* growths (Hyphae) on any surface in the aquarum looking a lot like lump of cotton wool in fact.
This is probably why this fungus is often called The Cotton Wool Disease.
- *Note: the colour may turn from white to grey to reddish-brown if allowed to advance.
When fungus attacks an aquatic animal then this is almost always due to a break in the skin of the animal which then then allows fungus spores to settle within and start growing, feeding on the decaying flesh of the animal.
This is called a secondary infection because usually bacteria is already present in the wound causing the flesh to decay and so providing a source of food for the fungus.
Other Types of Fungus
- Chytrid fungus - Affects Frogs.
- Mycorrhizas - A beneficial fungus used by plants.
- Animal injuries due to fighting.
- Knocks on sharp objects.
- Bacterial infection.
Commonly found on:
- The mouth of the animal.
- Gill flaps.
- Eggs of animals.
A healthy animal will naturally fight off a minor injury infection. But if the animal is stressed (as is often the case if it's a new arrival from another tank or shop) then its immune system may not be able to fight off an infection and the fungus grows and spreads. Eventually the fungus grows so much it interferes with the animals behaviour (especially if it's over the mouth) and can cause death.
Fortunately there are many excellent cures available on the market for the most common Fungus. However there are a few 'snail oil' treatments that have no proven record of working. Basically be aware that 'herbal' or tea-tree oil remedies as these are at best very weak in action.
The most common treatment chemicals called 'fungicides' are:
- Methylene blue
- Malachite green
- Gentian violet
- Sodium chlorite
- Various commerical bottles.
There is another disease which looks very similar to 'Cotton Mouth disease and this is actually a bacteria infection called Mouth fungus (it is not a fungus, it is the bacteria Flexibactor columnaris. So to treat this you'll need a bacterial treatment specifically for this.
It is difficult to tell the two apart but generally F. columnaris has a coarser, more granular appearance and can be a dull grey-white than true fungus.
The basic four treatments used for decades are : Salt, Methylene blue, Malachite green (or Acriflavine though is this now getting withdrawn as it is toxic to humans)
For years this has been one of the favourite methods to treat fungus as it is cheap and relatively effective. Basically for freshwater animals, place the creature in a quarantine tank which has a slight salt content. See Salt article for detail. Basically ensure the species is able to tolerate a salt bath, not all animals can and you'd be better to use a commerical fungicide instead if you are unsure.
Make up a 0.1% salt tank and place the animal in it. Increase dose by 0.1% every 4–6 hours until you get to 1%. (If at any time the animal looks distressed then dilute the salt content by performing water changes) Leave the animal in the bath for 12 hours at 1%. Then start reducing the salt content slowly by performing water changes over 12 hours until the water is cleared of salt.
This is a dye used in chemistry as a blue staining agent. It has been used for years for destroying fungus, bacteria and parasites. It is often supplied as a powder or as a pre-prepared liquid and is relatively safe if used correctly.
- See Methylene blue article for more detail.
This potent chemical dye is used as a 1% solution (typically use 1g of powder dissolved in 100ml of distilled water) and then dapped directly onto the infected area of the fish with a cotton bud daily until the infection is healed.
Use 0.5% solution if treating tetras or small fish.
- Note: This chemical is harmful to humans!
Gentian violet is a water soluble dye (colouring substance) used primarily in medicine to stain bacteria. Treat like Methylene blue.
Phenoxyethanol is often used as a fish sedative or anaesthetic but also has antibacterial and fungicide properties. This chemical is used as a 1% solution (typically use 100 mg of powder dissolved in 1000ml of distilled water) and dosage is 10ml per litre of quarantine tank water. Repeat every 2–3 days up to a maximum of 3 times if the animal shows no signs of recovery before performing a 50-60% water change.
Used by a few commercial bottles and incorrectly named as 'Stabilised Chlorine oxides'. This is a bleach so the bottle solution will be diluted. Be very careful with dosage when using this chemical.