Fish have nostrils called nares that lead to sensitive organs of smell so fish can find food. The nostrils are not used for breathing purposes.
Fish move water quickly over their sensory pads to pick up chemical signals. The sensory pads send these signals to the brain, and the brain then interprets them. The brain gives the fish chemical cues that allow it to take action, depending on the information. The signals serve as an alert system for both food sources and nearby danger. Gills are what the fish use to breathe; gills take in water and absorb oxygen from it.