Causes
There is much debate as to the cause of HLLE. Very little scientific research has been done on the topic, and most information available is anecdotal.
Removing carbon from the filter is a commonly suggested remedy for the disease. It is suggested that the carbon may either a.) add fine carbon particles to the water that irritate the skin, b.) leach potentially harmful chemicals into the water, or c.) remove minerals that are important to the health of the fish. Though there have been many reports of fish being affected by carbon, only one study has substantiated this; http://www.coralmagazine-us.com/content/hlle-and-activated-carbon-looking-link
Some people believe that parasites of the genus Hexamita, a flagellated protozoan, is to blame for this syndrome in freshwater fishes. HLLE and Hexamita infections are often seen in the same specimens. However, HLLE can be found in many fish who do not have a Hexamita infection, suggesting that a Hexamita infection may cause stress or interfere with the absorption of vitamins and minerals causing malnutrition, which may be the actual cause of the disease. Hexamita may also be a secondary infection common in fish already weakened by HLLE.
One non-replicated study showed that improvement in nutrition will help symptoms most effectively (Blasiola 1992). Key vitamins in preventing or curing HLLE seem to be Vitamins A, C, and D. Adding these vitamins to the diet of affected fish usually leads to improvement, however, deficiencies in any of these vitamins do not always lead to HLLE, so nutrition is also questionable as a cause.
It has also been suggested that HLLE is an autoimmune disorder that is triggered by stress. The disease does not appear to occur to fish in the wild, only those in captivity, supporting the idea that stress and unnatural living conditions are to blame. Anything that reduces stress appears to help in the prevention and recovery from this disease.
Read more about this topic: Head And Lateral Line Erosion