How do goldfish die?

Usually a sick fish has its dorsal fin down rather than fully extended. Other things to look for are a sunken belly, any kind of whitish, stringy or fungussy (like cotton wool) stuff attached to the fishes body, mouth or fins. Scales should be laying flat smoothly with none missing. Splits in fins are another thing to warn you of problems as are whiteish spots about the size of pin pricks on the body and fins of a fish. Another thing to watch for is the way the fish swims and positions itself in the water if it seems to be struggling to either go up or down in the tank it has problems. There are heaps of other symptoms of illnes but the ones above will probably help you a little.