Well.. yeah. Basically you don't really need to run the canister filter if you have a protein skimmer and a sufficient amount of live rock(which is 1 - 1.5lbs PER gallon of water). However, it certainly won't harm your tank to run a canister filter. I see TONS of reefers and non-reefers using canister filters today to run carbon, GFO
, zeovit, and other things you couldn't run if you didn't have a sump. It's another good way to keep your water clean, but don't think it is necessary.