The pattern of death is parasite, one. then another, spread much slower than bacterial infections. So I would use a quinine based parasite medicine, which I just did to my 29 with corydoras, and they quit dying. Finished meds about a month ago. I can't say what they had, I can say they aren't dying anymore.
Clear Ich was a powder you mixed with water, common 30 years ago, mfgr is gone but it was the best stuff. And it was less troublesome to use than the stuff I just used. (which seemed to drop pH - it was supposed to be used 10 days with daily water changes - I did 5 - it was adequate. Clear Ich was a 3 or 5 day run and it worked back in the 90s.
What I used this time was Quinine Sulfate from National Fish Pharmaceuticals, but they are wholesale only and it comes in a really big jar. I could send you enough to dose with if you want to try it, and remind me how big your tank is. But it's a white powder and you are outside the US. I'm not sure it can go in the mail.