Keep them in a set-up similar to what people keep similarly large and messy tropical species (like large catfish, cichlids etc.) in and you'll be fine.
So a large and just as importantly wide tank, with oversized filteration. Because they are coldwater and very messy (they are just a type of carp), you may need to provide an extra circulation pump or two on top of a couple of large external filters.
Something over 4 foot long and ideally not much less than 2 foot wide would do, as mentioned goldfish easily attain over a foot in length very quickly when healthy.
They will require weekly, probably twice-weekly water changes to keep them healthy and growing when they are larger too! You may decide to donate them to somebodies garden pond one summer if you get tired of the water changes, but at least then you can use their old tank for smaller tropical fish, and be amazed at how much less work they are to keep lol!
Thanks I'll keep that in mind, I'm doing my search on filters, so hopefully, they'll be happy
My friend is already planning to eat them when they grow bigger lol, so evil! (No, that will never happen!)
