I reckon it is a bit of both but more towards the shop because the tank should have at least mentioned that it was a marine setup and the store rep shouldve asked what setup they had also they should have asked what kind of fish they wanted.
Not necessarily. This is a shop, not a school.
If I go into a car dealership, they don't first ask me to make sure I don't want a motorbike before selling me the car I want. It is taken as given that you are able to do some research yourself and know what you want. They also don't ask me to make sure my drive/garage is big enough to accomodate the car.
Last time I bought some light bulbs they didn't ask me to make sure I really did want bayonet fitting.
It only seems to be fish stores that we bemoan for their lack of giving us all the information and making us undergo the Spanish Inquisition before they offer to sell us anything. But then if one of the stores says something that is a little different to what people on the hyperweb say, loads of members here will jump on the board and moan about their "stupid lfs".
You are buying an animal? You are responsible for it, therefore you are responsible for researchig to make sure it is suitable.