Oxygen and Water Movement

Fish breathe oxygen — but not from the air. They breathe oxygen that is dissolved in the water, collected through their gills as water flows past. The tricky part is this: water can look perfectly clean and still be dangerously low in oxygen. A crowded tank, a warm tank, a dirty tank, or a tank with no water movement will run out of dissolved oxygen faster than you might expect. And when that happens, the fish — as Ozzie will tell you at considerable length — cannot breathe.

Good oxygen levels need good water movement. A working filter, an air pump, a spray bar that ripples the surface — anything that keeps the water turning over and exchanging gas with the air above.