I agree with alot of what is being posted here about this problem. Espcially about making sure that water quality is correct and that a good diet is being fed. I also agree that in this case, attempting to medicate the fish would be the last thing I tried. I would only medicate if a couple weeks of excellent water quality and diet didn't improve the condition.
The thing I don't agree with or understand is all the suggestions for administering medications that treat external parasites. Head pitting and laterline eurosion is not caused by a parasite feeding on the fish's skin. It is most often a reaction the the enviroment it is in or nutritional defecencies. Copper treatment is known for actually causing head pitting and laterline eurosion. THis is most often seen in salt water fish but is true for fresh water as well. Medications for external parasites is not going to fix the pitts in your fishes head or fix what caused it. If your fish have an internal parasite such as heximeda, the medication must be soaked into the food you feed them to get the meds into the digestive tract. Fresh water fish do not drink water, having medication in the water will not get into thier digestive tract.
I'm not trying to step on anyones toes here. I have delt with this condition many times and treated the condition as a external parasite with very little + outcome. I started treating this condition as I discribed above many years ago, mostly on discus, with a much greater success rate. The garlic trick works suprisingly well, even on external parasites. Kent marine and Sea Chem both sell a garlic extract to soak your food in.
Joel