Chinese restaurants, if in Asian countries, such as Hong Kong, Macau or China etc, with a big fish tank at the entrance of the restaurant, probably means you can choose and buy your own seafood freshly and order to be cooked in the kitchen there the way you like, such as steam , fry, stew etc.
If a Chinese restaurant in the western world such as Europe or US with a fish tank in it, it mostly is a Feng Shui thing. One of the rule of Feng Shui is , a fish tank can bring wealth to you. No matter where it is, be it in a home or in a restaurant. It also depends on where you put the tank and what you put inside the tank. Mostly they will put a kind of golden carp in it and which is a very expensive fish.
Having said that, it doesn't mean that Chinese restaurants in Asian countries do not believe in Feng Shui, a fish tank in a Chinese restaurant in Hong Kong can just be for Feng Shui purpose. Depends on what's inside the tank. If you see lobster, king crab, prawn and all kinds of fish inside, it means they are for selling on the menu cause Feng Shui fish tank will only have fish in it.