I bet plain old raccoons got the goldfish. Depending on how deep they were willing to work in. My father in law had a little waterfall and catch basin and we put goldfish surplus from my pond in there. As long as the waterfall action was running the coons didn't tend to try to go in the water to get the fish. But when he put the waterfall on a timer to go off at midnight to save some money, the coons cleaned up very quickly. It must be that they can't find the goldfish when the surface of the water is slighty agitated?
We caught it on game camera, one big fat raccoon cleaning up on goldfish. The other idea besides running a bubbler or agitator is to add a ledge of stone that goes out over the water that the fish can get under and the coons cannot. Or make a fenced in area with small openings that only the goldfish can retreat into. They are more likely to go in the fenced area if it also is shaded.
