Why does the sea water in the aquarium take water from the bottom of the sea?

Will there be natural seawater in the aquarium of coastal cities? Unfortunately, many coastal cities have aquariums, mostly in estuaries or inner bays. There are many factories, farms and waterways nearby. The water quality is turbid, and the pollution is serious. Red tides often break out, and fragile coral reef organisms can't bear it. Sometimes, the cost of filtering such polluted seawater is higher than that of preparing artificial seawater with fresh water. So coastal metropolises such as Shanghai and Guangzhou simply built the aquarium not far from the sea, using artificial seawater. In order to make profits, the aquarium needs as many tourists as possible, so it will not be built far away from the urban area except for some specially large ocean parks. However, if there is a relatively clean sea area nearby, it will be much easier. You can lay pipelines on the seabed to extract natural seawater from there. The reason for taking water from the sea bottom is that there are a lot of plankton and impurities on the sea, and the water quality in the deep sea is relatively clean.