goldfish tank

Hello guys! I want to, eventually, sell my 11g for a 29g. I want to stock it with however many black goldfish would fit inside. You know, the ones with the really big eyes?? I used to have one as a child and I want another sooo bad. Of course- I would wait until my current fish children...

I am thinking about getting a Black Moor Goldfish. I was wondering what size of tank would be best and if a filter would be needed? At first I was thinking a 5 gallon tank with an air stone, but I’ve seen several people saying they must have a 20 gallon at least? Would a 10 gallon tank be big...