Captain lives in 110 litres, on his own, with an oversized filter and a weekly 30% water change. He is a year old and about eleven centimetres, and he is nowhere near done growing. I mention it because I bought him in a 20 litre 'goldfish kit' that the shop sold me without comment, and the ammonia readings in month one were genuinely alarming. If you are setting one up: cycle the tank first, buy the biggest one you can fit, and ignore the kit.