Are you sure? They might be "gasping at the surface" or searching for food. My platies have started doing that, and taht's my take on it (I think there's enough O2 in my tank, though).
The gasping at the surface can also be a symtom of disease or toxins in the water and not just low oxygen levels. Many diseases and toxins reduce the amount of oxygen in the blood even if the amount of disolved oxygen in the water remains constant. You may want to double check your ammonia and nitrite levels just to rule them out.