Any kind of a canister filter or hang on power filter is eventually going to be breaking down your ammonia and nitrites into nitrates and pumping them into your tank. Which equals more frequent water changes. A skimmer can slow this process down considerably. It removes the stuff before it goes into the break down phase, and thus, less nitrates and less water changes.
Bak-Pak and Aqua C are two of the best bangs for your bucks out there. By all means fit one into your budget.