It is not only Goldfish, it is every fish.

It is not only to have a tank with water and put the fish in.

It is about keeping our pets in an environment where they are happy and thrive this means we must provide them their environment to be as natural as possible. And because it is a closed biological system, stagnant water, not flowing and exchanging and cleaning itself like in nature, it is up to us to make up for this.
This includes to monitor the water quality, perform regular waterchanges with conditioned water ( chlorine and chloramine removed, beneficial bacteria added, pH adjusted if necessary ) provide excellent filtration and aeration.

Sounds  complex  and  "difficult"  but it isn't 
Basically the bigger the tank, the less "time consuming" it is. Easy to look after

5 other goldfish?! Are you buying a bigger tank so you can crowd it even worse than the current one?
No, if you can, please decline the other goldfish, because it WON'T WORK. You'll destroy the whole purpose of getting a larger tank for your current fish!

but the tank owner can't put them anywhere

Poor Tubi, her tail is still handicaped

I'm getting it at the beginning of the november. I think I can make the arrangements

Maybe you could move the smaller fish to the smaller tank or something? And let the goldfish have the new, bigger tank?
Or what kind of fish are those that came with the 100L?