Andrink, welcome to the forum.
As I understand, homes in Hong Kong usually do not have as much space as is common in the USA & some other countries. Perhaps this is true for you, and the reason it is difficult to get a very large tank. Also an influence with the fish stores advice on size.
You might give some thought to switching your fish to something that will do well permanently in the tank size you can keep. These decisions aren't always easy.
Happy fish keeping!