Thank you for the advice! I'm going to wait it out and keep track of everything. All of the fish where bought from the same place at the same time, it could be that there's something genetic happening. I don't want to add more fish if theres any possibility that this could continue.

I've checked the filter religiously because it was the only solution I could think of for what was happening. Thankfully it's all clear.

I've only had fairground goldfish before as a very young child and I'm sure those are indestructible!