Fungus is as the name tells us caused by fungi (Saprolegnia) and will only attack weakened fish that has already been infected by another disease. Fungus will never attack an otherwise strong and healthy fish and the best way of avoiding this disease it therefore to keep your aquarium clean and your fish healthy and well fed.
Fungus will eventually kill infected fish when it has spread through the fish even if this often takes quite a while to happen. The symptoms of fungus are gray or white growth on the fins and skin of the fish. The gray or white areas grow larger and more cotton like as the disease progresses.
Fungus is fortunately rather easy to treat and there are several different ways to treat it. One is to add methylene blue to the water. Methylene blue are most commonly used to prevent fungi on fish eggs but can be used on adult fish as well even if it isn't necessarily the best option for adult fish. Personally I recommend using methylene blue for treating and prevent fungi on eggs and fry. On adult fish I instead recommend using a phenoxethol solution or commercial antifungal medicine designed to treat fungus in fish. If you choose to use phenoxethol solution you should add 10ml of 1% phenoxethol solution per litre aquarium water. The treatment can be repeated after a few days if needed but should not be done more than twice since repeated use can have damaging side effects. Severely infected fish can be caught and swabbed with a povidone, iodine or mercurochrome soaked cloth to increase the effectiveness of the treatment. Make sure to increase the water quality and fish health in your aquarium once the fungus has been cured to avoid the problem in the future.
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