Such fish can be found in pet stores as "feeder fish"; live food for larger fish to eat. I kept good care of mine, changed their water often, but they didn't last long anyway. I was told "gold fish don't live very long." But it got me interested in keeping fish, and got me started with my first 10 gal. tropical fish aquarium.
Since then I've always been interested in aquarium fish. In the early 1990's, I tried keeping goldfish again, fancy goldfish, just three of them in a 15 gallon aquarium, with good filtration, etc. Despite my best efforts, they didn't last all that long either.
Many years later; I'm living on a small farm. We have a 300 gallon tub, it's made to hold drinking water for livestock, but we just use it to hold rainwater or creek water for watering green house plants.
During the summer, it becomes a breeding pool for mosquitoes. We usually add anti-mosquito biscuits to the water, but this past summer, we bought a bag of "feeder" goldfish instead, and dumped them in to eat the mosquito larvae.
The water got murky, so we used an old pump and set up a charcoal/ammonia stone filter up in an old cat liter bucket, with a roll of floss-like material in it.
It worked quite well. The water stayed clean and clear. We took buckets of water out to water the greenhouse plants, and occasionally replenished the tub with water from our creek. We fed the goldfish flaked food for goldfish. They got very tame and friendly. There are about 16 of them now.
We figured the goldfish would croak by the end of summer, but they haven't. In fact, they seem to be thriving and getting bigger.
So why haven't they died, like all the one's I've kept in aquariums? Well, it seems that by keeping them in a large filtered tub, and taking out part of the water and replenishing it on a regular basis, we were inadvertently providing the ideal conditions for keeping goldfish.
I've been reading up on goldfish, from many different websites. It seems that goldfish aquariums require frequent partial water changes, of 30% or so, every few weeks. Even if the water is filtered! This is because goldfish have no stomaches, only an intestine. Thus they produce a lot of waste and foul the water quickly. Filtration alone will not prevent buildup of certain toxins over time, requiring water changes.
I also found out that Goldfish DO live very long, 20+ years typically, 40+ years rarely, if cared for properly. The following site lists several points worth noting about the proper care of goldfish:
Goldfish (Carassius auratus)
[...] Goldfish should NEVER be kept in unfiltered bowl environments. This is not a suitable home for any living creature.
Fancy Goldfish need at least 75.7 Litres (20 US G.) per Goldfish and Common Goldfish need at least 208.2 Litres (55 US G.) per goldfish. If well cared for, Fancies can get around 20.3cm (8") long and Commons over 30.5cm (12"), so adequate room for swimming and turning is also necessary. For this reason, Commons do best in a pond environment.
* It is myth that Goldfish only grow to the size of their tank or that there are slow growing varieties.
Goldfish are large and messy fish, so you should have filters that move at least four times the amount of water in the tank per hour. Very high flow rates greater than seven times tank volume may be harmful as some varieties of Goldfish are weak swimmers. Filters with separate areas for mechanical and biological are best. [...]
I was shocked that they suggest 55 gal. PER FISH, for a common goldfish. But those little feeder fish are only small because they are babies. According to one video, they can grow quite large in just 3 years:
Yikes! The large one is a foot long! They are in a 75 gal. tank. Some sites I read said you only need 20 gal. for the first fish, and 10 gal. for each additional fish. But perhaps that's for the fancy ones, which also get big, but are still smaller than the plain ones.
So now I know why my "feeders" have lived; they got plenty of room, and regular water changes. But what's next? Am I going to have them for 20+ years?
Who knows? It's early days yet. They have yet to survive an Oregon winter outside (but I believe the pool is deep enough to allow them to survive the types of freezes we have). The cat, and local raccoons haven't discovered them yet; but it's a deep pool, so they could have a hard time accessing it.
What I DO know for sure is, that they have already lived longer than they would have if they were sold as fishfood for larger fish, as they were meant to be. So whatever length of life they have now is gravy. I will keep taking care of them, and enjoy them for however much longer they last.
For more Goldfish FAQ and advice, also see:
Common myths about goldfish