Get yourself a 75 at least. You've got a common and they need 75+, honestly the best tank possible to buy would be a 120. Your guy is easily going to get 12 inches and they like to have friends, who should also be another common goldfish. I've seen these tanks on sell at petsmart before but you also have to add in the cost of the filter and everything needed. It's a pain in the butt. OR you could go a cheaper route and build a pond or buy a swimming pool (https://www.amazon.com/Bestway-First-Square-Wading-Swimming/dp/B00DYVEWX8/ref=sr_1_39?s=toys-and-games&ie=UTF8&qid=1542224851&sr=1-39&keywords=swimming+pool something like this would work). But depending on how cold it's going to get that might not work out best for you, though that last one could easily be set up in the basement and moved out in the summer. Best bet is probably to find another home.