[Is a 20 gallon sustainable for a fantail?]

Wow such amazing comments and help thank you all I will consider everything!

[Is a 20 gallon sustainable for a fantail?]

So I live with my parents and the biggest tank I can have  is a 20 gal tall (I have other smaller tanks) and I have a good sized fantail goldfish not adult size but still big. I was wondering can I keep him in that for another 2 years cause that’s when I’ll probably move out. Right now I have a big sponge filter with a big hygger duel sponge filter and an internal filter and I do 50% water changes once or twice a week. I love this fish and don’t want to get rid of it I just love goldfish.

Oh and I have plants