Salt Water Aquarium II

I was thinking back to the saltwater aquarium that I saw yesterday and I became mindful of yet another fluid mechanic principle at work. The fish in the tank possessed a capability to obtain neutral buoyancy at any depth. This is because fish have a unique organ, called a swim bladder, that acts similar to a ballast tank on a submarine. Air can be added or released from this organ. When more air is added the fish maintains the same volume but changes its density. This allows the fish to rise in the water column without having to expend energy swimming there.