Do not use a credit card if it is on the rocks, that only works if it is on the glass. You need to peel it off slowly using your fingernail. Go slowly and gently, they are very delicate and tear easily. If you have a hard time, just transfer the rock it is on into the new tank. If the rock is too bit you could use haevy duty rock snippers and cut it off. I guess it depends on the type of rock you have, some are softer than others. Good luck, I hate removing them from the rock, it's nerve wracking.