skimmer /need?

[mytank90]

this might be a wierd question .i have a 180 gallon tank with roughly 120 gallon sump with about 20 mangroves in ref.i have about 400 lbs live rock total in system with roughly 15 medium to large fish .i run a large bubble king skimmer on system.my prblem is i do not get anything from skimmer .the bubbles stay clean of if lucky i get a realy lite color .if i have enough rock do i need to run skimmer ? i have tryed useing my eshopp skimmer from a smaller tank that i know pulls gross stuff but even it will not pull anything from tank .i have though to take the bubbble king out of sump and add little more rock but sould i?