Body fungus will rapidly kill aquarium fish, and it will not clear up on its own without rapid treatment. Several treatment options exist, and they can completely clear up the fungus if you catch it early enough. Additionally, you should take steps to prevent it in the first place.
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Body fungus is a true fungus, caused by fungi from the family Saprolegniaceae. More specifically, various fungi from the related genera Saprolegnia and Achyl cause most cases. The fungi appears as long, white fluffy strands growing from a fish's body; it can also grow on the head and fins. The strands usually have a white color, but debris in them can give them brown, reddish or green colors. It will typically kill fish within the week, though this can vary.
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You have several treatment options for body fungus. You can use antifungal drugs -- available at pet shops -- to treat the condition. Follow the manufacturer's instructions to the letter. A home method for treating is adding 1 teaspoon of kosher, rock or sea salt per gallon of aquarium water.
Unfortunately, some fish, like tetras, are sensitive to antifungals. Antifungal drugs may also harm the bacteria that live in aquarium gravel and break down fish waste. At the same time, some fish, such as cory catfish, do not tolerate salt well. For this reason, you should isolate the infected fish in a separate quarantine aquarium for treatment, since body fungus does not easily spread between fish.
You can easily prevent body fungus. The fungi that cause this disease are considered "opportunistic" infections. This means that the fungi lives in most aquarium water, and even on the skin of most aquarium fish, without causing disease. However, these usually harmless fungi can gain a foothold and cause disease in fish who suffer from stress or injury. To avoid stress, do not overcrowd or overfeed aquariums. Isolate fish that bully others, to prevent injuries. Also, keep up on water changes to make it less likely that fish will suffer from body fungus.
One other common aquarium disease resembles body fungus -- mouth fungus. To make matters confusing, both disease can infect the body, head and tail, though more often than not mouth fungus starts on the mouth and body fungus on the body. Mouth fungus typically has shorter tuffs of fuzz. However, mouth fungus is actually caused by a bacterium (Flavobacterium columnare) that has fuzzy-looking colonies. Treatment is similar, only with antibiotics in the place of antifungals. Like body fungus, this bacterial infection responds to salt, does not readily spread between fish and can be prevented with good aquarium practices. With successful treatment, mouth fungus typically clears up in about a week.