Paragraph:Hole in the head or also commonly known as Hexamitiasis is a parasitic in which lesions will appear over the head of the fish and can cause holes through the fish's head (these can be as small as paperclips and get large up to the size of a finger in width). In most cases, there will be erosion not only around the fish's head but also in various spots around its body as well. Since a vast majority of fish carry this specific parasite in their intestines by nature, this only will show symptoms when the fish's immune system becomes weakened by either another disease or due to poor water conditions.
Any fish that is infected with this disease will produce white, stringy poo and their body coloration will go from being vibrant to a paler in appearance. As this parasite affects the fish, even more, they will gradually eat less and less until they become emaciated. The final stage of this results in the fish showing somewhat holes in their head (or in their body) which may appear as if there is a complete hole through, although if caught early it will not create a full hole. This commonly infects cichlids and those fish that have large, thin heads, however various carp related fish such as goldfish and koi can be infected just as easily if any of the fish who carry this parasite have their immune system weakened over a long period.
Hexamitiasis is caused by the protozoan parasite genus Hexamita, which is present in the lower levels of the intestines of most fish (as they do not harm that location). Only when the fish's immune system becomes extremely weak is when the parasite can multiply and spread to the rest of its body. Sadly, as the parasite infects other key organs the fish will have no other fate than death (even with treatment). Since a large majority of fish already carry this parasite inside of their intestines, it is not technically considered to be contagious to others who are not infected by it.
Be aware though that just the fact of a fish having lesions or erosion on its head or body is a case of Hexamita, in some cases, malnutrition can cause the same symptoms which lead many to believe that their whole tank (or a group of fish) are infected.
Treatment and Medication
The primary medication that is known to treat this disease is metronidazole (often commonly called Flagyl). By using this medication in the infected fish's food will kill off the parasite from internal to external (thereby forcing the parasite out of the fish's body where it will die in just the water column). Another least effective way when the fish is refusing to eat would be to dose the medication inside of the water column (following the bottle's directions for exact dose requirements is necessary).
Due to the way that this disease spreads and affects the inhabitants, the best prevention methods are to always make sure that you quarantine anything that you place inside of your water (this includes any plants, inhabitants, decorations, and more). In many cases, this can also result from the condition of poor water quality and/or the health of the fish that are brought into the aquarium. Thus, when you do let them adjust to your water and you truly see how they are acting, eating and if they have any symptoms or signs, we will either start to get a better understanding if they are infected or learn about what possible other diseases they might have.