What is Fungus?
The two main types of fungus in aquarium, we class them into "True Fungus" and "False Fungus".
False Fungus ( Columnaris Disease/Mouth Fungus/Mouth Rot)
While this disease look like a fungus attack, it is actually a bacteria infection. As with most common diseases, false fungus is caused by stress in the living environment of the fish.
In the long term form (chronic form), they appear as light grey markings on the fish., lips of the fish or inside the fish's mouth. These markings then become fluffy and look like cotton wool, therefore making people think they are fungus. This false fungus has a more coarse and grainy look, and they are more gray than true fungus. As this false fungus develops, the skin appears more and more of these ulcers. The places that get affected is usually the mouth and gills because they are more vulnerable but it can infect anywhere.
There is another form in which this occus, and in this form the disease strikes hard and fast. There is no external symptoms and the affected fish can appear listless and die just a few days later. A post motem examination is the only way to discover this infection.
Both of the above forms of fungus are caused by the same bacteria called Flexibacter Columnaris, which is why this disease is also called Columnaris Disease.
Attacking False Fungus
External false fungus/columnaris can be treated with potassium permanganate at a concentration of 2ppm for 8-10 hrs. As with any treatment, fish should be observed closely and if there is any signs of stress, you should do water change immediately. After treatment, you need to do a complete water change because potassium permanganate is harmful to soft tissues like the fish's gills and you need to clear it out so as to give the fish a chance to recover. If you can find phenoxyethanol, you can also use it to make a bath to immerse the fish in. The advantage of using phenoxyethanol is that phenoxyethanol will also treat both the false fungus and true fungus so if you make a wrong indentification it works too.
True Fungus (cotton wool disease)
They are whitish and fluffy like cotton wool, usually occuring at the site where the fish has a physical injury or abrasion/wear and tear. True fungus is caused by stress in the living environment of the fish and usually very bad conditions like poorly circulated and aerated water or rotting uneating food in the gravel bed or filter.
Attacking True Fungus
Since this is a disease caused by bad water conditions, fix the water! For the affected fish, you can remove the affected fish to a hospital tank to treat separately. You can give your fish an extended salt bath as treatment, but it takes time. Don't be expecting the fungus to disappear immediately or in a short time!
You can also use medication containing Phenoxyethanol if you can find it. For best efect, you can apply the medication onto the infection directly using cotton bud.
Unlike most other diseases, fungus attacks is hard for the fish to recover itself once the infection has gained a foothold. Treat the fish and pay close attention to it!