Not all sick goldfish can be treated but if you can identify the disease and act quickly then you have a good chance of being able to save your goldfish.
When a goldfish is sick it will show signs in its behavior such as:
• clamped fins
• shaking body
• constant scratching against objects in the tank
• hiding in the corners
• gasping at the surface
• swimming abnormally
With the exception of wounds and some visible fish parasites the treatment of sick goldfish takes place in water either in their own tank or, if possible, in a separate hospital or quarantine tank. A separate treatment tank will allow you to administer the correct dose of medication as you should be able to easily calculate the volume of water. If you use a hospital tank it should be aerated but activated carbon should be removed from any filter as it will absorb the medication.
Before returning your fish to the main tank or pond remove the medication from the hospital tank by doing partial regular water changes. This will assist to acclimatize the fish to the clean water conditions of the main tank.
Your hospital tank won’t need plants or ornaments although you might like to add a couple of plastic plants to give the fish a feeling of security.
See the comprehensive article on how to treat sick goldfish for additional information