Fancy goldfish can be almost anything, as Jenste has implied. I made the assumption that you knew the minimum tank size for your fish although the standard that we live by says the first goldfish, type not specified, takes 20 gallons and each added one takes about 10 gallons. That would mean 30 gallons, about 115 litres, for a simple pair of comet goldfish. If you are rearing common goldfish to a large number, please be aware that you are well outside the experience of most of us here. We are mostly tropical fish fanciers and don't know as much as perhaps we should about goldfish.