Meh, I wouldn't upgrade a 40 breeder unless it was a 75. The dimensions of 55 and 60 are vomit to me.
Confuses the hell out of me why they make large tanks tall and narrow. What is it for, a billion nano fish?
Once my fish get to 6 inches each, I'll upgrade to a 75 with a 2nd canister.
And on the 75, I'll do same with 40, no plants. I grow bamboo using a Aquaclear 50 with nothing in it but ceramic biomedia and the bamboo, on a very slow flow. The filtration is from a Fluval 407. Future plan for next year when upgrade to 75 is to add probably another Fluval 407. For the 75, probably upgrade the Aquaclear to a 110 to do the exact same thing, just add more bamboo 😄
Also I think goldfish are seriously the most underrated fish. They are adorable, and constantly moving, doing weird things. One of my Jikan spent the afternoon swimming downward against the flow of my air stone. And I didn't know they were such sociable fish, they are constantly following each other, bumping into each other, and just being clowns.
I know with larger fish tanks, people really are focused on cichlids or carnivorous fish. But I think fancy goldfish may just be the best fish for a tank from 40-75 if you are just keeping 3-5 of them. Don't do 4, 4 remember = death.