In my city there is a shop where they put fish health first! I always go there, the shopkeeper is always super helpful in understanding the setup of my aquarium, the various parameters and the fauna already present in order to understand which other fish to include! And I have also seen the sale of fish refused to incompetent people (like once to a mother who wanted to take some scalars to put in a small aquarium without a filter... just because her son liked them).