Seems the hardness was a red herring. Several people with endlers in hard water doing perfectly fine.
This latest one is still with us. Hiding a bit but often swimming about perfectly well with the others.
He looks like a shark with this wierd indentation under his chin. Still haven't seen him eat the food I put in, but he's still fat and seems perfectly well otherwise. I don't think this is the same thing the others had, they were dead by now. So maybe that was worms of some sort and the med got them.