Oscars, as well as other large cichlids, are commonly afflicted by hole in the head disease (HITH). The symptoms are easily recognizable pitted areas or holes around the face and head area and along the lateral line of the fish but the cause of HITH is not so clearly understood.
Some aquarists blame poor water conditions as the cause of most fish problems. This fact emphasizes the importance of a good filter system, regular water changes and aquarium maintenance. Another factor could be stress on the Oscar that weakens his immune system. The source of the stress could be many things bullying, overcrowding, inadequate tank size. By observing how your Oscar behaves you can readily see if he is happy in his home or not.
A poor diet can also contribute to HITH. Humans need a balanced diet with enough vitamins and minerals to be healthy and so do your fishes. If you only feed your Oscar feeder fishes he will very likely contract HITH as well as other problems. Others have also suggested that carbon may be the cause of HITH but this theory has not been proven. Hexamita (an intestinal parasite) was believed at one time to cause HITH but this idea has proven to be false.
Luckily, treatment is both easy and effective. First, begin a feeding program of vitamin enriched foods and supplements which can be found in most pet stores. Next, make regular water changes of 20-30% every three days and remove any visible waste matter daily. Then, add 1 tablespoon of aquarium or kosher salt for every 5-10 gallons of water in your tank and raise the temperature to 82F (remember to replace the salt that is removed during the water changes). Within a few days you should see an improvement in the Oscar’s appearance. After the holes have begun healing and shrinking in size the temperature can be returned to its normal range and water changes can be done weekly. Also, the amount of salt can be reduced to 1 tablespoon per 10-15 gallons.
Once your Oscar is well, remember to maintain your fish’s health by providing a clean, stress-free home and a varied vitamin-rich diet.