There is something called hole in the head disease but I don't know much about it. I caused by a parasite - API general cure claims to be able to fix it. I don't know if they sell it in Norway though or what medication would be equilivant
General Cure™ Powder
Treats a wide variety of parasitic diseases including velvet, anchor worm, fish lice, hole-in-the-head disease (Hexamita spp. & Spironucleus spp.)
, gill & skin flukes (Dactylogyrus spp. & Gyrodactylus spp.). For use in both freshwater and saltwater aquariums. This medication will not color the aquarium water.