Thanks for advice. I’ll try peat when setting up a bigger tank somewhere in spring.
After consulting various other forums, someone suggested my fish may be suffering from gill flukes. The symptoms hinted at either those, ammonia or nitrite poisoning or lack of oxygen. Water tests dismissed the second possibility, and even though I deprived myself of sleep by placing an air pump beside my filter’s spray bar (the tank is next to my bed) the little one still gasped. I tried treating with trichlorfon (metrifonate), which should get rid of most parasites but there is hardly any improvement. The fish isn’t gasping as much, but still moves its gills far too rapidly. It still shows no interest in food. Today I also noticed its anus is red. The same as the late nitrite victim.
EDIT: The little one set off on its last journey
I really hope I'll figure out what went wrong without loosing any more fish