I had to take my goldfish out of a 20 gallon tank and put it in a garden tote cause he couldn't turn around in the aquariums easily anymore.
I got a round garden tote, ya know the one with rope handles? It's quite large, but because it is round Dexter can swim around it with no problem. Dexter is 10 years old and quite big now. LOL I put him out on the patio in a half oak barrel with a liner with a pump in the summer time and then just bring him back in the house in the fall. I use to put him in my pond, but he got too hard to catch and my pond freezes solid in the winter time.