Goldfish, as a species, are one huge
design flaw. In the most literal sense, the first goldfish were
mistakes that would not live long enough in the wild to reproduce consistently with each other. Goldfish began when a spawn of a species of grass
carp that were normally muddy brown occasionally threw a
mutant with bright
bronze scales that reflected the sunlight and made the normally
dull fish brilliant and easy to see. Fortunately for them, most humans thought them
attractive and caught them to keep as pets, keeping them from being eaten like their brown siblings by humans or being easy targets for predators. But that's
ancient "goldfish". Today's goldfish carry on the
tradition of being flawed in design thanks to
man, not
nature. Only the plain "comet" goldfish that are sold as feeders, ten for a buck, resemble in any fashion their
carpish ancestors and the natural durability of the species.
Goldfish are now bred for show and decoration and as many fancy mutations as possible. And most of these "beauty marks" lead to inbred fish that could never, ever survive in the wild. Just a few examples:
The lionhead has been bred for a huge fleshy growth on its skull, reminiscent of a lion's mane. This is extra tissue, much like a wart on humans. This particular mutation also has been bred to have no dorsal fin whatsoever. This leads these fish to often be "weighted down" by their own heads and stand motionless, noses to the gravel, until something happens that makes it worth their time to move (usually food is about the only reason) because it is physically exhausting for them to swim unnecessarily.
The bubble-eye has been bred to have sacs of fluid under its eyes. A thin balloon of skin--no scales--gradually develops on these fish and fills with fluid as the fish ages. However, this skin is very very delicate and will burst quite easily when brushed against anything sharp or abrasive. Sometimes the sac heals and fills again, sometimes it does not and leaves the fish lopsided. In many cases, the burst bubble kills the fish, not because the bursting hurts the fish much, but because it is an open wound and a secondary infection will quite readily destroy the already delicate breed.
The pearlscale looks like a swimming golf ball with a very round body and very thick scales, low on the edges and thick and high in the middle, looking like seed pearls collected on the fish. These fish often have so much difficulty swimming that it is terribly detrimental to their health and they will rest, stomach on the gravel, most of the time. It is also nearly impossible to tell if they have parasites, a common problem in goldfish, because they are so bloated and misshapen around the stomach area anyways, any increase is difficult to detect.
Other flaws in goldfish are not so breed specific, but include huge flowing finnage that makes swimming terribly difficult for the fish, "telescoping" eyes that are more easily damaged, enlarged bodies which can increase swim bladder problems, and other, more minor, issues. The long and short of it is, nearly all fancy goldfish are incredibly impractical, improbable creatures that need a bit of extra attention to insure their health. They're marvelous little animals, but they don't make sense.