You will not need a skimmer to start until your system cycles. Now having said that, if you intend to cycle your system with uncured live rock, a skimmer is a must, if you intend to have corals, fish, deep sand bed, live rock, etc, a skimmer will most likely be requried. if you intend to put one or two corals and maybe 3 or 4 small fish in your system and religeously do water changes and maintenance you may not need a skimmer. a skimmer will make things much easier for you and give you some forgiveness for the occassional over feeding or dead animal. hope this helps.