Is that a white spot on its tail too or is that just the light?
I think it looks like a case of ick: https://thegoldfishtank.com/goldfish-info/diseases/goldfish-diseases-white-spot-ich/
I know you say your other fish don't have it, but it is infectious so they may do soon if I'm right.
I'd love to keep a pet rabbit or two one day, but I live in the countryside and I've always been too worried about foxes or other predators getting too them.
Is it safe? Or can you keep them in the house instead?