I wish that mine would exhibit that kind of growth.My Irwini catfish puts on about an inch a month, growing way quicker than my Granulosus cat.
I think the key to their happiness is not disturbing them and giving a good hideout. Mine has a sweet spot under a few flat rocks and a giant piece of wood. I can access the tank from the back, so I drop food down to the back of his cave. He plows thru algae wafers and hikari pellets like crazy.I wish that mine would exhibit that kind of growth.
You forgot to say "and provided with enough space and clean water"Goldfish also grow very fast when they are fed a lot.