oh wow, yeah a 12 inch stone is way too big for a 10 gallon tank. Keep in mind that 2 goldfish will definitely need a bigger tank though. What kind of goldfish are they? Being extremely dirty fish and the fact that they can grow quite large, they would be better off in a 20-40 gallon tank for the two of them depending on what kind they are. A smaller tank could become dirty rather quickly and will stunt their growth.
If its a new tank you should definitely read up about the nitrogen cycle. Its a cycle that will have a large effect on your tank for the first 4-6 weeks and is a process that continually goes on in your tank. Its a good thing to know. You should be able to find posts about it here or articles online or just ask and one of us can explain it
good luck and welcome to the forum.