Your Goldish is suffering from a bacterial disease called Hemorrhagic Septicemia which is essentially internal bleeding. The fish is also suffering from a disease called Fin Rot which is also bacterial in nature.
The way to treat this is:
1) Change all of the water in the fish's bowl.
2) Add a pinch of tonic salt as you have done.
3) Begin to treat with a medicine called Interpet Anti Internal Bacteria No9 which can be found at pet stores and online. Click on link below for a view of the medicine. Follow the directions on the product label for usage/dosage instructions.
4) Keep the fish isolated in the seperate bowl for 14 days whilst changing all of the water every 3 days whilst adding a pinch of salt.
5) During the 14 day quarantine feed the fish only a small pinch of food one a day.