Depending on the size of your goldfish you could theoretically still keep turtles without posing any significant danger to the fish. I always suggest getting a hatchling to up to 4 inches and introduce it in the middle of spring so they are not "crazy" from winter hibernation or simply from hunger induced by temp increases. Maybe a Mississippi Map would work well as they are pretty small and tend to stay that way as long as you get a male and don't overfeed him. Also don't underfeed or he might eat your fish. If you are seriously interested in making it work and really want advice to get a turtle send me a private message and I will be glad to help out with advice etc. as much as I can.
