You could get by with these 2 fish in a 20g for some time, just shoot for larger weekly water changes. A real basic rule of thumb for fancy goldfish is, 20g for one, with 10 additional gallons for each additional fancy. 29g is a nice easy to get, off the shelf tank size for this. They can often be had for $1/gal on sale at Petco, Pet Supplies Plus or sometimes Petsmart.