Yes, fish essentially breathe water through their gills, but they don’t use the water to make energy, but the oxygen that is dissolved in the water. In essence, fish are like us except they use a different mechanism to get their oxygen! Fish also don’t need as much oxygen because they are cold-blooded, so they can ‘afford’ to get their oxygen from the water. Some people boil the water for their fish tank before putting it in to get rid of chlorine, so if you do that always make sure to let the water sit out overnight so that oxygen can re-dissolve into the water, otherwise your fish will suffocate! Fish exhale carbon dioxide through their gills, but it dissolves in the water too, so you don’t see bubbles.