Oxygen is essential for the well being of all life and that includes aquatic life. Water adsorbs oxygen from any point where it contacts air. For this reason a 10ft by 10ft pond having a surface area of 100 square feet will absorb oxygen from 100 square feet regardless of its depth and so the fish stocking capacity of a pond is relevant to this calculation. Disturbing the surface with the use of air stones or turbulent filter returns such as venturis or waterfalls will increase the surface area because instead of a flat surface the ripples being made produce a wavy surface. If you can imagine measuring a flat line that measures 10cm long ---------then compare this to measuring a wavy line ~~~~~~ also 10cm long the ups and downs increase it to maybe double the length. If you try this with a piece of string you will see what I mean.
Lack of oxygen will cause fish to gasp at the surface especially in the early morning. Some unexplained fish deaths often at night when plant life is using up more oxygen than during the day are symptoms of lack of oxygen. Larger fish which need more oxygen to survive along with fish such as orfe and rudd (surface swimmers) which are very sensitive to oxygen levels will normally be the first casualties. Water can also develop an unpleasant smell and appear tainted.
Improving oxygen levels is done by adding features such as waterfalls, air stones, squirting ornaments and venturis. Reducing the load on your ponds system is done by ensuring, stocking levels are not too high, adequate filter size and maintenance, removal of excess algae and blanket weed and reducing the quantity of plants in the pond.
During warm weather especially humid weather caused by thunder storms it is vital too maintain adequate aeration. This is because warm water contains less oxygen than cold water and humid weather also reduces oxygen levels.