If you have a good skimmer you don't need to filter. If you have a reef tank you want to have small particles ( microns in size) to feed your corals.
I have a 155 with a 35 gallon sump, 2- 250 watt MH's with Luminarc reflectors and one actinic 96 watt PC. I don't use any kind of filtration or carbon. Live sand and live rock with a reef concepts skimmer is all I use. I have softies and SPS and all are thriving. I use Randy's formula for additives.
A good skimmer, even though it may cost $600, is worth every penny. I've gone through a few poor ones and will never go back.
Other than magneting the glass and water changes, this tank, though bigger, only takes 1/10th of the time to do maintenance on compared to my previous tanks. If I added up all the costs of old filters and gear on old tanks, $600 would have been cheaper! Not to mention the time spent cleaning.