Oh dear... I will tell you from my experience... I had a goldfish named
Poko. He grew to be over 10 inches long and was 11 years old. For the
last few years of his life he had a tumor on his eye. It wasn't big at
first, it just looked like the eye was clouded on the outside. But
over the years it got bigger and bigger.

It was really gross, but he would hit it on something and then it would
fall off and it would grow back. It got really big and I was getting
worried about it. I did actually find a vet that would remove it, but
by then it was too late for him...

I had him in with a loach and the loach ended up eating the tumor off
his eye... and ate his eye. Poko spent his last day flopped on his
side, and eventually died from the pain.

I don't know what you can do about it, really. I had asked many fish
stores and they had no idea. I don't think medications can help, I
think it can only be surgically removed. Surgery on fish sounds silly,
but it is available. I'm not sure how much it costs though. And
finding someone to do it may be hard. The closest place to me that
would do the surgery was an hour away.

Best of luck.