is there such a thing as swim bladder disease?

It's not really a disease, but more of a disorder, or sometimes a symptom. It's cause can be any number of different factors. I've heard that stunting in fancies such as orandas can cause it, because their body style makes for crowded organs under the best of circumstances. They can also display it with any number of illnesses, or even a case of over feeding.
 
Birth defects are very possible. So yes, one could be born with problems with the swim bladder, although in that case it wouldn't really be swim bladder disease. Others are more prone to getting it.

Swim bladder disease is more of a symptom like dropsy or popeye. Many things can cause it. The big two are water quality and round-bodied goldfish eating floating foods, which causes them to take in too much air, which can lead to swim bladder infections. The duct from their mouth to their swim bladder is usually not as open as it should be, this is what makes them much more likely to get infections there.
 
Goldfish are pond fish and should be kept in large ponds. The fact that your cousin said 100 gallons doesn't make him an idiot at all. We are talking about a fish that should reach 16-18 inches (fancy's not counted of course). Any tank that is less than 18 inches deep is not suitable for long term health.

I'm pretty sure not all goldfish variations reach more than a foot. I believe some stay around 7-8 inches or so.
 
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