Fungus-

True Fungus and False Fungus

The two main types of fungus in aquatic life can be separated into the "True Fungus" and the "False Fungus".

False Fungus
(also called Mouth Fungus, Mouth Rot, or Columnaris Disease)

While this disease exhibits all signs and symptons of fungus it is in fact a type of bacterial infection. The cause is stress conditions such as overcrowding, injury, and/or poor water quality with low oxygen.

Symptoms

Chronic form
Small patches of whiteish to grey markings on the head, but sometimes also on the fins and gills. The lips and the inside of the mouth are the most likely to be infected due to wear and tear,
The initial lesions develop into fluffy growths looking much like cotton wool, thus looking like fungus. Mouth fungus (false fungus) has a more coarse and more granular look, and is usually more greyish in colour compared to true fungus.

Acute Form
An infection which has an incubation of a few days, usually happens at 25oC-32oC. Infected fish may have no external symptons and just die in a couple of days after appearing listless. Many "unexplained fish deaths" in home aquariums may be due to this infection, as diagnosis is usually only possible on a post-mortem examination.

Both forms of False Fungus are caused by a bacteria - Flexibacter Columnaris

Transmission
This is a highly communicable disease. Lesions usually first appear on the caudal fin and spreads towards the head. The caudal fin and anal fins will get severely eroded. As the disease spreads, the skin is often appears numerous gray-white ulcers. Damage often occur at the gills and it may be the only affected area. (Gill lesions are characterized by necrosis of the distal end of the gill filament which progresses basally to involve the entire filament.)

Prevention
Monitor your aquarium for correct water parameters. Use an air pump to increase water circulation and dissolved air. Change water regularly when water conditions deteriote.

Treatment
External columnaris can be treated with potassium permanganate at a concentration of 2 parts per million(ppm) for 8–10 hours. This concentration will cause the water to be wine red in color. (If the color fades to yellowish-brown in less than 4 hours, it may be necessary to retreat). In a tank, potassium permanganate can be used at a slightly higher concentration, but fish must be observed during the treatment. As much as 10ppm can be used for a short bath of 30–60 minutes. Water should be changed immediately if fish show signs of stress on close, careful observation. Following treatment, a complete water change is necessary to avoid gill damage, giving the fish chance to recover.

Bath immersion using a treatment containing Phenoxyethanol is also normally effective. Phenoxyethanol is fungicidal as well so it will also treat true fungal infection.

True Fungus
(cotton wool disease)

Symptons
Fluffy and whiteish growth, looking like cotton wool. They are usually at the location of physical injuries caused by fighting, parasites or abrasion. Sometimes untreated diseases can also result in fungus. If left untreated, they may turn grey to brownish with time, as they collect dirt and algae.

Transmission
True Fungus spreads if left unchecked to surrounding healthy tissues, leading eventually to mortality. They usually occur only in places with poor water quality. In an aquarium, uneffective fitration systems can cause uneated food to stagnate and rot, increasing the chance for fungi to gain a foothold.

Prevention
Monitor your aquarium for correct water parameters. Use an air pump to increase water circulation and dissolved air. Change water regularly when water conditions deteriote.

Treatment
You do not have to treat the tank, if it is not really needed.

If few fish are infected, extraction into a hospital tank is a better way. If all the fish have fungus, you have to treat the whole tank but remember, treat not only the symptoms but more importantly, the source.

If your fish and plants are salt resistant, you may use a prolonged salt bath.

Add an aquarium fungicide, such as one containing Phenoxyethanol.

If you do not have a hospital tank and the fish is still quite healthy you can apply the fungicide on the affected spot with a soft swab.