Depends on tank size and livestock. Skimmers and canister filters do NOT perform the same functions. If your tank is a reef with LPS or SPS corals over 30g I reccomend a skimmer. If its fish only over 30g and you overstock and/or stock with messy predators then I also reccomend a skimmer. About the only times I reccomend no skimmer is for all tanks under 30g, for lightly populated fish only's, and for softie tanks.