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To stop sinking, fish needed the equivalent of buoyancy tanks on a submarine. Less gas and we sink, more and we rise. But whereas a sub uses compressed air, a fish must carry its gas supply in its bloodstream. For this, fish have an onboard float - the swim-bladder that is a large gas-filled sac that inflates and deflates as needed. Gas diffuses back and forth between the swim-bladder and the bloodstream, letting fish float effortlessly at any depth. Despite being heavy boned the fish is, in effect, weightless.