Add water. It may simply be running with too low of water level in the back and the fallling water is creating bubbles that get sucked into the powerhead which acts like a blender making a froth of bubbles.
Sometimes a pinhole leak in the piping connections can draw in bubbles that get shot out of the nozzle.
Also, realize that as waves crash in the ocean there are lots of air bubbles sometimes down as far as 10 feet as well as all kinds of food organisms traveling in the water, especially at night.
Edited by Mark Peterson - January 17 2010 at 9:54am