If someone ask about goldfish on our forum (here in Fin), we recommend to buy at least 300 liters tank for 2-3 fishes. If you buy more, then larger tank. Good filtration but you have to do something to "flowing". If you buy "natural" goldfish, we recommend to buy larger tank already at beginning, because goldfish grow fast. Goldfish isn't either "good fish for beginners", that you have probably heard several times. It isn't true. Goldfish needs more caring than normal tropical fishes..
Goldfish is a messy fish. It eats alot and several times a day (small stomach). You also need to change water often. Every plants aren't good to keep with those fishes. Some of them last, most are eaten.
Goldfish also lives 15-20 years, those mutant creatures.. "Normal" goldsfish can lives couple decades easily even 30-40 years.
How many goldfishes, you know, have lived more than 6 months? People uses to buy goldfishes as easy fishes and put them into small tank (or even into fishbowl!). They are still large and need space, but no way, those people just love to watch llarge fish in a jar and enjoy it. (sadistic!)