I thought I had already replied to this but don't see my post.
Although ribbon snakes eat fish, it would take a really big one to take a bigger goldfish. My guess is you have been paid a visit by an egret.

I had a friend years ago who lost a group of expensive koi in one day. He wondered if somebody had stolen his fish. Later a neighbor told him he saw an egret 'messing around' in the guy's pond. The neighbor thought it was kind of interesting to see an egret in the yard like that.

We saw snakes around our pond during the drought but none since. None large enough to eat our huge fancy goldfish. We were visited by a great blue heron this past Fall and Winter. I built a pond cover from pvc pipe, plastic grid and zip strips. He has finally given up and no longer visits.