You hardly ever see Head-and-Lateral-Line-Erosion (HLLE) in the wild. It is almost exclusively seen in some marine aquariums. Now, thanks to a study performed by Jay F. Hemdal, the Curator of Fishes and Invertebrates at the Toledo Zoological Society, we now understand the major reason for this. Do you have one or more fish (probably a surgeonfish) showing signs of this disease? Here is the reason why and what you can do to prevent HLLE
More: Hyposalinity or Osmotic Shock Therapy is a safe and effective treatment for a number of parasite infestations. Before you start spending money of chemical treatments, you might want to look into it.