But that might not be the cause of the smell. If its only flake food causing the surface layer increase surface aggitation as mentioned, and feed sparingly. Your water params seem to suggest no waste, but if you have a smell, I fail to see anything else that would cause the odor. Only feed however much the fish would eat in a few minutes, 2-3 times a day. Increase your cleanup crew if you find food sitting on the floor of your tank a while after feeding. Anyone else from the brains trust got an idea as to what could smell from a marine tank, if not the water?
Edit: You could always stop feeding flake, to see if the smell goes away. Use frozen.