You need a tank, a sump, 1 to 1.5 lbs of live rock (or DIY rock) per gal, heater, pumps for at least 20 times per hour turn over of your water, lights for the type of corals you will be keeping (I like T5s), and a skimmer rated for twice your tank size. All the rest are add-ons to make your life easier, they can come later. (JMO)
