Aquariums: What could this be?

I already gave a treatment with Levamisoli hydrochloridum and Praziquantel, it seems better but not everything is gone. She can't close her mouth like her sister. She is now breathing heavily since 14 days.

Once in a while other a platy or a guppy lose weight, breath heavily without noticable symptoms and die very fast. I Lost 6 fish in 3 months.

Only the last fish I bought from a hobbiest have problems, my oldies are all fine.

Parameters 7pH, 10GH, 6KH, NO3 10. I do a weekly water change of 10%

I have this tank 8 years, 60liters.

I'm about to buy a microscope to do research. I really want to find out what is going on.