I wouldn't attempt, espescially if you are a busy person who can't change the water frequently.
The general stocking rule is 20g for the first (FANCY) goldie, 10 for each additional one
However due to variation in breed size, some breeds of goldfish (ie Telescopes (including the black moor), Pearlscales, and Bubble-Eyes) require closer to 10g per fish wheras some of the larger breeds (Ryukins and Fantails) may need closer to 15 or even 20g per fish.
While the 20g high (which is what I'm keeping my girl in til the 50g comes!) may be suitable for one fish, goldies are social (and smart!) and do best in groups of at least 3.
If you are willing to shell out for a tank of at least 30g though I strongly recommend it! They're hilarious fish and will even eat from your hand. Fancies will live from anywhere between 8 and 20 years and make a good 'pet' wheras most of my fish are just a 'hobby'. Mine are gonna be with me all through college and longer <3.
PS. If you're still not convinced, here is some eye candy
to get you started... I didn't think twice about goldies until I saw what this guy had!
And lastly here is my first (so far) goldie...
I named her Satsuma (an chinese sweet orange). Not my photo skillz though this is the breeder pic lol!