I think my fish might stop dying now. They were dropping one by one from whitespot, but I've been treating the pond with anti-whitespot stuff and it seems to have cleared up. There's one fish with fin rot that's probably a secondary infection from whitespot damage, so I've put it into a hospital tank in the greenhouse. I was expecting the whitespot treatment to mess with the pond's ecosystem, but all the arthropods and tadpoles and things are fine.