Are pet shop fish, better off in tap water?
You’ve got fish in your pond and they are fine? That rules out dodgy pond water. Sounds like you have good tap water, but that’s a variable. Mine changes to another supply 1 mile from my home and they add fluoride. My PH is 7.0, there’s 7.1. Tap water quality is a lucky dip. The history of your fancy will be the cause of the problem. It’s never encountered the germs that live in a pond and has no immune system. That’s to say it was born and raised in tap water. Not to say this isn’t curable, you just have to expose it to the pond water at a much slower rate, so it builds up a natural immunity.
