goldfish grow to a certain size depending on the size of their tanks. the bigger the tank the bigger the fish. they need at least 10 gallons per fish PLUS 5 gallons per inch of fish. so ONE 5-inch goldfish would be 35 gallons required.
goldfish can grow over a foot long, but this is only under proper conditions in an obnoxiously large aquarium or in a pond and takes many years to accomplish.
in your 20-gallon you can afford 2 baby goldfish. and once they are around 2-3 inches id get a bigger tank, and maybe get new baby goldfish for your old 20 gallon! =)
comet goldfish and koi typically grow larger than fancy goldfish, because of the way they are designed (fancies are bred to have 'attractive deformations' which sometimes lead to health problems down the road) however i find fancies much more entertaining.
Comets and koi easily reach over a foot long in a pond, meanwhile most fancies plateu at 8-11 inches (typically, they CAN get much larger. bruce was a fancy goldfish and he was 16 inches long. u can google him)
dont ask the ppl at the pet store how many u can have in ur 20 gall, they five times outta ten are more willing to make a sale then give a fish to a good home. some stores will give 5 goldfish for a 5 gallon tank, and all those fish would easily die in a week. 2 would be max for a 20 gal. seriously, for the sake of the fish. =/ if you really want a more interesting tank go for cool decor and plants just dont overstock it.
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