Hole In The Head Disease

Can also be known as Head and Lateral Line Erosion (HLLE) and is a fish disease that can affect freshwater and marine fish, especially those in captivity. Hole in the Head may be used in reference to the freshwater variety and HLLE to the marine.
Symptoms

As the name suggests, the fish will initially present with small pinhole erosions around the eyes and head, these are a greyish colour. If untreated these can become larger and spread around the head and along the lateral line.

Causes

It has been suggested that hexamita, a small flagellated protozoa is responsible for the disease. Hex meaning 6, refers to the 6 anterior flagella or long thread like appendages that are used as a means of locomotion. Hexamita infections can often be seen in specimens exhibiting symptoms of the two diseases. However, some fish with either Hole in the Head or HLLE do not show signs of Hexamita and this may lead us to believe that if a Hexamita infection is present it may lead to stress in the fish which can in turn weaken the immune system. Lack of the absorbtion of vitamins and minerals can then lead to malnutrition giving the disease chance to take hold, and in fact may be the cause of the disease. Thus, hexamita may be a secondary infection in already weakened fish.
Very little research has been done into the cause of the disease and there are still open opinions as to it's cause. Some suggest that the removal of carbon from filters will go some way towards helping as they believe that fine carbon particles in the water may cause skin irritation, that it will cause phosphates and other harmful chemicals into the water and also that it may remove certain minerals that the fish need for optimum health. Again this has not been substantiated as aquariums which have carbon in the filters still maintain healthy fish.
Improvement in nutrition may help to relieve the symptoms, this has been shown in various individual studies. Vitamins A,C and D have been seen to prevent or even cure cases of HLLE and the addition of these vitamins to the diet of fish in the early stages of infection will usually be seen to improve them. Conversly however, a deficiency in these vitamins does not always lead to the onset of the disease, so once again nutrition cannot be held as a definite cause.
As fish in the wild do not appear to suffer from HLLE another suggestion is that it is a disease of the auto-immune system caused by stress. Captive fish may develop it due to the unnatural living conditions they sometimes have to endure. Therefore, anything that minimises stress may help the prevention of the disease and also aid in it's recovery.

Preventative Measures

moving fish to a new aquarium or pond, where practical improvement of filtration and more frequent partial water changes enriching the diet, especially with vitamins A,C and D including green foods in the diet removing carbon from the filtration system paired with upping partial water changes feeding chopped garlic use of a salt bath to improve the slime coat and prevent secondary infections treating with medication for internal parasites or secondary infections maintaining optimum and constant pH levels relevant to the type of fish you have.
After the first few days of treatment the fish should satrt to show signs of improvement and regain energy. The damaged tissues should begin to repair. The above measures should be seen as a change in the care of your fish and as such, a permananent change in husbandry.