Just a short update: I have just added the 4th and last recommended dose of meds. I decided on Furan-2 in the evening and tetracyclin in the morning. The fish are still alive, although I have to say that I don't see any change in the symptoms. The guppies eat as always (they also ate until they died before), and the tetras lost appetite.
I used the meds according to the direction, and the result looks truly disgusting. The tank water is a deep, murky green from the Furan-2 and covered in a foam layer from the tetracyclin. I can still see the fish, though the water is dark and not very clear.
I'm not sure whether I should stop with treatment now and try something else or go on, at least with the Furan-2.