The goldfish bowl is an outdated fish torture device. Learn how to care for these fish properly.
Step 1: Tank size
For (long bodied) common/comet/shubunkin goldfish you will need 200 litres (50 gallons) for the first one and 80 litres (20 gallons) for each additional.
For fancy goldfish (fantails,pearlscale,celestial etc) you should have 80 litres (20 gallons) for the first one and 40 litres (10 gallons) for each additional.
Long bodied goldfish and celestials should be kept in a species only tank.