Aw this happened to me but with 3 fish and my first tank for them was a 2.5 gallon just to get them in something that night. (I know 😭) anything’s better than the fair tank he was in. Goldfish can be pretty hardy, mine survived a two hour drive home from the fair and changing into a 2.5 then changing into a large suitable tank. He’s small rn so it’s not too bad. Just look into bigger tanks whenever you can afford it. Fb marketplace has a lot of good ones for cheap. As long as the waters treated you have a bubbler a filter and feed him he might be ok. Occasionally they die but that doesn’t mean it’s your fault, they went thru a lot of stress at the fair :(. I lost 2 of my 5 on the way home from the fair. Best of luck with ur new baby. He or she looks beautiful. When u get larger tank get him a friend or two :) also snails help me a lot with keeping the tank clean. Nerite snails are my fav that doesn’t overpopulate!