Jerzy, could you please answer some of the questions asked by the others? I'd rather be reading complete details to see what I am missing here.
Kzimmerman is right though. When I read this yesterday, I had an inkling flukes may somehow be involved or at least any ectoparasites do. I'd get the salt and prazi in there. Both are very mild treatments and should be conducted to every new fish during quarantine process to keep invisible ectoparasites at bay. There are instructions for dosing prazi and salt in "List of Treatments and Contraindications" and "Freshwater Disease Guide-Ich" sticky threads.
Kzimmerman is right though. When I read this yesterday, I had an inkling flukes may somehow be involved or at least any ectoparasites do. I'd get the salt and prazi in there. Both are very mild treatments and should be conducted to every new fish during quarantine process to keep invisible ectoparasites at bay. There are instructions for dosing prazi and salt in "List of Treatments and Contraindications" and "Freshwater Disease Guide-Ich" sticky threads.