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they breathe water
Via the gills, by osmosis. The oxygen they need is dissolved in the water.
Fish extract the oxygen out of the water using their gills, thus leaving the water hydrogenated, which some plants and animals (marine ones) need as opposed to oxygen so balance is restored. Hope this helps.
Fish breathe underwater using their gills.
Fish "breathe" by gulping down water. This water passes over the fishes' gills, made up of the same flesh as our lungs (roughly), which extract the water using either active transport, or simply a concentration gradient on a partially permeable membrane, I'm not quite sure. Either way, the oxygen is extracted from the water, and Carbon Dioxide replaced. The water then passes out the slits in the side of the fishes head.

This method is very effective, meaning the fish can extract around 80% of the oxygen in water, which is EXTREMELY important, as water only contains 2-5% oxygen, as opposed to 20-something% for air.

Also, the ease of which the oxygen is extracted depends on the water temperature, whether it is salty, and other factors. This is why most Freshwater Fish can't survive in the ocean.

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