While the medium may be different, the actual gas molecules being exchanged are the same because fish also use aerobic metabolism. Fish also breathe oxygen, but must extract it from water that passes through the gills (as opposed to say, mammalian lungs, that must extract oxygen from air in the lungs). What they breathe out is also carbon dioxide, but usually it is excreted in a water soluble form (bicarbonate, or HCO3-).
Hence, while humans breath a mixture of gases (air) into and out of our lungs, fish have water flowing across their gills. As this water flows across, the fish must extract oxygen and excrete carbon dioxide.
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EDIT: And to expand on whoever introduced this earlier...yes the mechanism is still diffusion. In water, oxygen has a lower abundance than in air, whereas carbon dioxide maintains a similar solubility in water as in air. Therefore, oxygen levels are what water-breathing animals are focused on. However, note several physiological adaptations in fish that make gas exchange efficient in fish.
In fish, water flows unidirectionally and is nearly continuous. Compare this to the lung in which the air within the lungs are expelled back along the same path (this is called tidal ventilation), and at any point you are either inhaling or exhaling. There is also a lot of dead space in the lungs that do not allow for full usage of the volume available. The flow of water across gills and the direction of blood flow within the gills allows for a counter-current exchange, in which the two fluids are flowing in opposite directions. This sets up a continuous diffusion gradient, allowing for efficient gas exchange. The lungs do not have this feature.
Within the blood, the gas-carrying molecules are still generally the same, with the important one being Hemoglobin. The biochemistry here is all the same.
Also note that fish are not warm-blooded and by being relieved of the need to maintain a set temperature, their oxygen requirements are therefore decreased.
Hence, while humans breath a mixture of gases (air) into and out of our lungs, fish have water flowing across their gills. As this water flows across, the fish must extract oxygen and excrete carbon dioxide.
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EDIT: And to expand on whoever introduced this earlier...yes the mechanism is still diffusion. In water, oxygen has a lower abundance than in air, whereas carbon dioxide maintains a similar solubility in water as in air. Therefore, oxygen levels are what water-breathing animals are focused on. However, note several physiological adaptations in fish that make gas exchange efficient in fish.
In fish, water flows unidirectionally and is nearly continuous. Compare this to the lung in which the air within the lungs are expelled back along the same path (this is called tidal ventilation), and at any point you are either inhaling or exhaling. There is also a lot of dead space in the lungs that do not allow for full usage of the volume available. The flow of water across gills and the direction of blood flow within the gills allows for a counter-current exchange, in which the two fluids are flowing in opposite directions. This sets up a continuous diffusion gradient, allowing for efficient gas exchange. The lungs do not have this feature.
Within the blood, the gas-carrying molecules are still generally the same, with the important one being Hemoglobin. The biochemistry here is all the same.
Also note that fish are not warm-blooded and by being relieved of the need to maintain a set temperature, their oxygen requirements are therefore decreased.