How so? it's a floating city in the middle of the ocean, meaning there's no way the Ark is going all the way down to bottom of the sea floor and when you look at the vid clip you can see light near the top penetrating meaning its not far down from water surface aka a aquarium pool. Only explanation would be the Resistance catch fishes from sea and drop in the aquarium for food storage perhaps.
Actually, they constantly refer to the Ark as an island, so it's possible they're settled on something (otherwise wouldn't they just float into land anyways).
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On the other point about the aquarium being empty, actually what they said was that their fish farms were empty, which may mean that they can't breed the fish they were using to eat anymore but may still have inedibles floating about.