Like I had mentioned befor you dont have to have a mech. filter but I would ,the more filtration the better.You mentioned you were confused about not needing a filter.Your live rock works as a natural filter as long as there is water flow through the rock hence needing filters or powerheads or both but if you chose just powerheads you will need frequent water changes RO water recommended by me the reason being then the water going in the tank has zero nitrates after going throuh a RO filter.They arnt cheep but you can get a decent one for about $200 on ebay new.A skimmer removes waste which will turn to nitrates if not removed.Carbon filters also help with waste removal.When I say waste I am talking about amonia and other debree that could be in the water amonia will turn to nitrates if not removed.Nitrates
are not a marine tanks friend you want your tank to be as close to 0 parts per million of nitrates as you can possiably make it.With high nitrates fish wont be as healthy and some wont survive even more delicate are coarls.Although there are some coarls that will do ok in mid level nitrates they will flourish and propogate in high quality water.So buy as much filtration as you can afford but first stock good live rock and a good protine skimmer.I ahve been keeping fish tanks fresh and marine for over 20 yrs so if you have questions I will try to help if you need.feel free to email me
theburrs2@charter.net