Because they are such hardy fish, people put them to extremes (e.g. a bowl) and the nitrate poisoning eventually kills them. However, if care is normal, they do seem to thrive extremely well, they are usually the last fish to go in an aquarium disaster.
Kayla, a goldfish is a much more needy fish than most people know. You see what are called Goldfish bowls and really most actual goldfish need 30 gallons for the first fish and 10 gallons a fish for each one after that to thrive. While they usually do not need heated tanks they do need a lot of care as they do produce a lot of waste. They are what are usually classed as pretty high waste producing fish and a lot of people just do not give them amount of space or care that they really need or perhaps the filtration to take care of the amount of ammonia that they are putting out and they eventually die from ammonia poisoning. It is sad but that is usually what happens. They think that because they are cheap fish they do not need much in the way of care or equipment. That is just not the case. When you take fish from their normal place in nature and put them in a tank you have to find some way of making it so their waste products are managed and that means either with a filter or some other means and usually it means a good deal of water changes.
Also it has been my experience that goldfish like bettas are some of the most overfed fish and since their water is not heated generally can become very plugged up with too much food and die from too much food being given to them. Another words they can be killed from kindness.
I am so sorry if you have lost fish that you care about. I know that this is a hard thing and we are here for you.
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