I have several fancy goldfish (2 ryukin, 1 oranda, 1 red/white telescope, 1 fantail) and about 100 fry in a small fry tank right now haha! (There's also a high-fin spotted pleco that lives in the tank with the adult goldies). (In the past, I have had celestial/bubble eyes as well). One of the best things you can do when setting up a fancy goldfish tank (as with most other tanks) is to make sure that none of your decorations have sharp edges that they can cut themselves on- this will help prevent infections/etc.
I feed my goldies frozen bloodworms and frozen brine shrimp (thaw them out in warm water with a pinch of Prime and then feed with Reptile Tongs). I also feed "low waste" goldfish flakes, and small snails (I breed my own and keep them in different tanks so I know they aren't carrying diseases,etc). Hope that helps!!
Also - they like hardy plants like Amazon Swords in their tanks. I would also say that with goldfish (fancy or not) the larger the tank, the better! They love room to swim and play! Oh and don't believe what other people say- they're no more trouble to take care of than any other fish really as far as cleaning goes- just do weekly water changes and clean them with a good gravel vac. As far as filtration goes- I rinse my filter every week and replace it once a month.
As far as fancy goldfish go- they prefer warmer waters than their comet cousins/etc. Even though they're fancy goldfish and not comets- they can still grow between 6-10+ inches, so make sure that they have a large enough tank! (At least a 20gallon/fish would help keep them happy)! I keep mine in a tank with a high-fin spotted pleco- so I try to keep the water temperature between 78-82 in my fancy goldfish tank.
If you have any questions, I'm only a pm away! Fancy goldfish are such fun little fish to have around!