Probably the biggest hurdle facing someone starting out in the marine aquarium hobby is exactly what equipment do require to get going?  Of course, everyone has their differing opinions on that subject and it isn't quite a simple as it might sound.  What it is required all depends on what is the goal are of the system.  Therefore, if you haven't pinned that down as much as possible, it is recommended that you work that out as specifically as possible before making any equipment purchases.  The reason for that is that the goal of the tank dictates the type of equipment that would be suitable.

Definitely Required

These are pieces of equipment that is definitely required to maintain an "average" marine aquarium with both fish and photosynthetic corals.

• Tank, to hold the water.  Most typical used is glass or acrylic.
• Heater, to maintain the water within the correct temperature range.
• Thermometer, to monitor the water temperature.
• Hydrometer, to monitor the water specific gravity / salinity.
• Pump(s), to move water around within the tank.
• Liverock, to provide biological filtration and a structure to place sensile organisms on (see: How much liverock is enough? ).
• Lighting, to provide sufficient light for the photosynthetic organisms to live and grow.

Handy to Have

This group of equipment is handy to have, makes life easier for you and will / may improve the environment for the organisms being kept, but isn't vital.

• Sump, to have a location to hide / place equipment out of sight (see: What is a sump?).
• Test Kits, to monitor (see: What test kits are required for a reef tank (or what parameters should I be monitoring)?)
• Calcium Reactor, to maintain calcium and alkalinity levels (see: What does a calcium carbonate reactor do? )
• Nilsen Reactor, to maintain calcium and alkalinity levels (see: What does a calcium hydroxide reactor do?).
• Protein Skimmer, to remove nutrients and waste from the water before it is processed by bacteria and improve water quality (see: What is a protein skimmer? )
• Fans, to maintain the water temperater in the correct range using evaporative cooling and improve gas exchange.
• Chiller, to maintain the water temperature in the correct range when fans are insufficient.
• Timer, to turn equipment on and off as required, most commonly lighting.

For the Gadget Freek

Generally these aren't needed for a successful reef aquarium.  However, there may be some circumstances where they will help out with things or will bring pleasure to those that love to have some toys to play with.

• Ozoniser, to produce ozone for injection into a protein skimmer, which helps with polishing water.
• Multicontroller, to automate the operation of the various parts of equipment.
• Fluidised Sand Bed Filter, to provide a large amount of biological filtration within a small area, good for heavily loaded systems and those without much liverock or substrate.