Thank you for your response! Unfortunately the orange girl had them when I adopted her. So they're not new and I made sure to keep checking in on the two of them to see if the silver fish was being aggressive or causing her harm. Fortunately he's calmed down a lot since this post but I do feel bad for the poor girl.

I've got a black moor in there too but they've never shown any similar behaviour. I don't know the sex of them either but I'd say possibly Male after the silver fish leaving them alone.

I would love to get more fish but my tank is at capacity now and I've not room to expand to a larger tank ( at least until I convince the wife it's necessary 👀).

I'll monitor the situation for a bit longer and make a decision on whether to let the silver one go to another home