To add to what Shawnie said, more severe cases might require medication. From oscarfishlover.com:Shawnie said:hith is an awful disease but curable...finding out how they got it and to not do it again is the first thing...water conditions , feeder fish, and lack of vitamins and poor diet are the main causes..although there are more causes, those are the two biggies..fresh daily water changes, stress coat + and some garlic juice will heal them up...but it has to be done daily for about 4 weeks....pooor fellas...hope they get better soon!
Relatively mild cases of HITH can be cured by increasing tank maintenance, and improving the fish's diet with maybe supplements of vitamins. However, severe cases of hole in the head may need medication. A lot of these medications are meant to be added to food but unfortunately, many fish that develop hole in the head disease go off their food so it can be problematic trying to administer medication. Luckily, there are medications on the market that just have to be added to the water. Two such treatments are dimetridazole and metronidazole. These two medications are only available as a prescription in certain countries so you will have to consult your vet. There are various other treatments available off the shelf. One such medication that is available here in the UK is called Octozin and is manufactured by Waterlife. These come in tablet form so all you have to do is add them to the tank water.