I don't see a surface skimmer on your filtration setup. Various organics in the water (fish waste, broken-down food, live rock die-off, human hands in the water, etc) get trapped at the tank water surface forming a "protein skin". Overflow boxes, integrated filtration systems and special filtration attachments are designed to pull this skin off the surface and put it through the filtration system. My hang-on skimmer has an attachment that does this.
It is very important to remove this skin as the tank water surface is where a large portion of your tank's oxygen exchange occurs. Lowered oxygen levels can really stress your fish. Having your filtration outlet or one of your circulation pumps set to agitate the water surface can help disperse the protein skin and improve oxygen exchange by increasing the water surface area.
Hope that helps.