I suppose all fish die out of water when their gills dry up and once they are dead their bodies start to change. So to be as fresh as possible before they are eaten they are kept alive in water, indeed there is a speciality dish in Japan where you pull the fish out of water, cut it up in front of the customer who then eats it while it is still moving.
The fish on the market stalls in Libourne were not kept in tanks, they were left flapping away until they suffocated on simple metal or wooden trays. There wasn't even any ice around them. I should add that they were mostly undersize flatfish which shouldn't have been caught in the first place.