Strange eye condition?

Hello all,

I'm hoping that you can help me as one of my goldfish has developed a
strange eye condition. Basically, he is absolutely fine in himself, feeding,
moving nicely, seems normal enough.
One eye has become very small and has only a very small amount of black
there now, whilst the other has become larger over time and the black middle
has become big. It's as if the one eye has gone over to the other side.
He seems to be able to get around really well and can identify food no
problem despite this.

Any thoughts on this would be most welcome.

Thanks all, Nick (UK)