Hole in the Head Disease, also known as Head and Lateral Line Erosion or HLLE, is a fish disease that affects both freshwater and marine fish in captivity. Sometimes "Hole in the Head Disease" is used to refer to the freshwater version of the disease, and "Head and Lateral Line Erosion" is used to refer to the marine version[1], though the causes and treatments for each are similar. Among freshwater fish, it affects cichlids and angelfish, with Oscars developing the disease more frequently than other related fish. Angelfish, tangs, and groupers, and to a lesser degree lionfish, damselfish, and clownfish are susceptible in marine aquaria.

Symptoms
The symptoms are well described by the name. First, the fish will develop small, grey, pin-hole abrasions around the eyes and head. As the disease progresses, it spreads down the lateral line. These lesions will grow and if not treated, can lead to secondary infections and may eventually kill the fish.[2]

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 phosphates and other 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 affected fish recovering after carbon has been removed, no studies have substantiated these theories, and many aquariums use carbon while still having healthy fish.

Many people believe that hexamita, a flagellated protozoan, is to blame. HLLE and hexamita infections are often seen in the same specimens.[3] 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.

Individual studies have shown that improvement in nutrition will help symptoms most effectively. 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.[4] 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.

Treatment
The prime medication used to treat hexamitiasis is metronidazole (often referred to by the trade name Flagyl). The most effective approach is using medicated food, especially when early infections are being dealt with. But seriously affected fish may not be eating, in which case you need to add the medication to the water, typically at a dosage of 250 mg per 10 US gallons, once per day for at least three days. Metronidazole is available to aquarists over the counter in the United States, but elsewhere a veterinarian prescription may be necessary. Alongside the medication, improvements to diet and environment are absolutely critical.

Prevention
Some preventative measures are outlined below.

* Moving the fish to a new aquarium, one that has never had this syndrome
* Improving filtration and more frequent partial water changes
* Enriching frozen or dried food with liquid vitamin supplements, especially A and C
* Including foods that are enhanced with vitamins, such as flake foods
* Including greens (fresh or frozen) in the fish's diet
* Decreasing the amount of beef heart as it lacks many critical nutrients and contains saturated fat which is unnatural in fish's diets
* Removing activated carbon filtration, and removing all carbon dust from the tank (Mangroves may be added or more frequent partial water changes may be used to replace carbon as a nitrate exporter)
* Reducing the use of feeder fish, as they contain little nutritional value
* Feeding chopped garlic
* Allowing the aquarium to receive direct sunlight[5] (whether that is because the light directly affects the fish or because it encourages the growth of beneficial microalgae and bacteria is unknown)
* Using products that replace the slime coat can prevent secondary infections and aid in healing
* Treating with medication for internal parasites or secondary infections
* Maintaining an optimum and consistent pH level for the type of fish you have
* Using Beta glucan [6]

The specimen should regain energy within the first several days of the treatment and the damaged tissue will gradually repair. It is important to follow the above steps as a preventative measure. This is not just a treatment plan, but a life style that encourages good health for Oscars and other susceptible aquarium fish.

I thought it was caused because of feeding them foods that do not have alot of protein like feeders. Most/all of the time for oscars it's because they feed them feeder fish and nothing else causing a deficiency in nutrients.