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