Yes, you can have one goldfish in a 20 gallon tank, but no other tank mates, not even a snail.
You still need to make sure you have a filter, and that you do regular weekly water changes as goldfish produce a high amount of waste.
it will live
Absolutely not. A goldfish needs at least 20 gallons to properly live and survive, and grow to it's real potential.
15 years if they are treated properly
No you schould not have that many fish in a one gallon tank. Thats the size of a big betta bowl. And because they are goldfish it is worse their poops are toxins
Goldfish as ornamental fish, come in a variety of shapes and sizes, of which the black moor is one.
they are found in collection tanks around the world but originally found in China
There is a popular bred of goldfish called a black moor. It has an egg shaped body, has protruding globe like eyes and is black in color.
None, goldfish should only be housed with other goldfish.
A black moor is breed of goldfish, similar to most others, if given the right environment, it would live to be around 6-8 years, but the oldest goldfish known is 49 years old, in case you're in an ambitious mood.
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Goldfish are opportunistic omnivores and will eat whatever they can. This may include a much smaller fish or a deceased fish.
The answer is yes if it's purchase size is 1 1/2-2inches For around four-six months a single Moor can live comfortably in a ten gallon. You will however need to upgrade to a 30 gallon after that time. If you desire to add another goldfish make sure it's healthy. Then you must add an additional ten gallons which brings you to a forty gallon, as goldfish consume so much O2 and produce a large amount of waste. Make sure that the 10g is equipped with a good filter and that the tank is cycled properly. Also, make sure you have a water test kit to test your parameters and do those water changes! Good luck with your fish keeping endeavors.