Just like any other creature; goldfish are immune to an array of various difficulties that may hinder with their normally calm living. Goldfish ailments aren't necessarily widespread, if preventative measures are taken to keep the fish's environment from becoming contaminated by bacteria, parasites, or other aquatic organisms.
Relieving goldfish illnesses will probably be totally impossible if they're being hassled by healthier fish. In addition to the fact that some illnesses can be contagious -- definitely quarantine ill looking fish pets in their individual container.
Bladder disease -- Signified by the fish floating on the top of the tank, or remaining extremely close to the bottom for long periods of time. This is much more frequently than not, a problem with gas and must be treated as such. When you were miserably bloated, you wouldn't be too fast to eat some more correct? This predicament is effectively served by simply not feeding the fish for a number of days
Lice -- Signified by green colored spots on the fish's skin. This is one of a handful of lethal goldfish conditions and your fish tank ought to be immediately medicated to stop death, or the dilemma spreading to other fish (if it has not already).
Dropsy -- Signified by an enlarged belly and swollen scales; probably the most lethal of all goldfish illnesses, it must be handled with quarantine and medication to become cleared up.
Ick -- This predicament is easy to diagnose, because your fish could have small white spots all over them. A cleanup of the tank, together with medicated drops or tablets of the water should clear up the "Ick".
Food For Thought
You can't constantly prevent problems from reaching the inside of your aquarium, but in most circumstances prevention is the most effective cure. If you supply clean water, a great filter and food for your goldfish, you should have very few difficulties. Almost all issues are triggered by filthy, untreated water and in several circumstances; as soon as goldfish conditions show themselves, they are harder to clear away.