Catfish can live only in water. Fish remove oxygen from the water with their gills. The more oxygen there is in the water, the easier it is for them to breathe. Nitrogen and oxygen are the most common. Air has plenty of oxygen in it.
Water has much less oxygen than air.
High quality water has 20,000 times less oxygen than our air. Parts per million and milligrams per liter are equal. One pound of oxygen dissolved in a total of one million pounds of water equals 1 ppm dissolved oxygen. Water temperature, atmospheric pressure and dissolved substances control the amount of oxygen water can hold. Cold water can hold more oxygen than warm water.