Hello, Michael,
that’s very sad ……….. Unfortunately, none of the experts of this forum seems to be online – no answer for the last 50 minutes.
So I hope my not so experienced advice is not totally wrong: To me it sounds like Oodinium. Peter Finke and Martin Hallmann in their new book give the advice to use Spirohexol (only tablets, no fluid) against Oodinium – although it’s a remedy against intestinal flagellates. I have tried it with my Badis, and it worked. It’s Spirohexol by the brand JBL. Hope I’m right with this advice.
Addition: As far as I know a slow (!!) rise of the water temperature up to 30 °C is also recommended.