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[Kaida]

The common rule of thumb for housing goldfish is that they need a bare minimum of 10 gallons of water per fish  for fancy goldfish (any goldfish having 2 tails) and a bare minimum of 20 gallons per common type goldfish (any goldfish having 1 tail).  In addition to all that space they require massive filtration.  To keep goldfish, you need a filter that turns over 10 gallons per hour (GPH) for every 1 gallon of water in the tank.  That means that if you have a 20 gallon tank you need a filter that covers 200GPH.

If you are keeping goldfish in any thing smaller expect problems.  I'm sure in your readings you've noticed someone mentioning the nitrogen cycle or cycle of an aquarium.  Basically the cycle is what happens when beneficial bacterias convert harmful ammonia and nitrite into less toxic nitrate.   All fish produce ammonia and goldfish make A LOT of it.  Somewhere in the ballpark of 250 parts per million every hour.  They make it when they poop, pee, and (believe it or not) when they breathe.  The same way we exhale CO2 fish excrete ammonia from their gills as a respiratory by-product. Ammonia is also produced by any rotting food the fish haven't eaten yet.  This is such a big deal because even small amounts of ammonia destroy blood cells and with out blood cells the fish cannot exchange O2 to it's tissues and vital organs.  Ammonia's converted form, Nitrite, is also harmful and has been proven to suppress the immune system in fish, leaving the fish open to attack from parasites and infection.  And while nitrites' converted form, Nitrate, is not as toxic as its chemical predecessors when it builds up and surges, as it does during the cycle establishment process, it destroys gill tissue and depletes the slime coat.

These are just the unpleasant short-term effects of poor water quality.  The effects of exposure to these chemicals are far reaching in fish as well.  Too often, fish that do manage to endure in these conditions suffer kidney, liver, and swim bladder damage that can not be reversed.  Their immune systems are permanently compromised, growth can be stunted, many become hyper sensitive to chemical imbalances, and their lifespans are shortened. 

Despite what the guy in the pet store told you, goldfish are not simple, little creatures that are cheap and easy to care for.  They grow quite large actually.  Common type goldfish can (and should) grow anywhere between 12 and 16 inches on average and fancy type gold fish usually range between 6 and 10in as adults.  Some do get bigger than that.  When you keep them in too small of a tank you stunt their external growth but often not their internal growth.  Stunted fish usually suffer from organ crowding which eventually causes vital organs to fail.

When properly cared for, these fish get huge and live long lives.  Some reaching well into their twenties although ten to fifteen is much more common.  They do feel pain and scientists have proven that they also display basic emotional responses (ie. fear, happiness, and even depression)  They are very social and even form close bonds with other fish.  Like any pet, they deserve to be properly cared for so I urge you to please research the needs of these animals.  You'll get so much enjoyment from them when they enjoy life.