The doors and most of the trim/fillers on this particular unit are solid MDF. But I have since gotten an automatic edgebander and do this type of construction with paint grade maple veneer (particleboard core) with all sides edgebanded, just because it is so much more water-resistant if the paint gets chipped. MDF is great as long as the paint doesn't get any gaps/cracks/chips that allow water to get in.

The main frame of this stand is the classic 2x lumber frame made popular by Rocketengineer on Reef Central's forums. Do a search there for more information on that.) Then I use my sheet goods to skin it over, but that's the hard part. it requires lots of planning and calculating, especially for the door hinges. Don't begin skinning the stand until you have purchased the hinges/opening hardware.

When I build stands and canopies, I use a hybrid of techniques derived from frameless cabinet construction. Personally I hate the 2x lumber frames because they make it so hard to get the doors to line up and adjust nicely, but with a huge aquarium there's usually no viable alternative. My own aquarium is a set of frameless cabinets with euro style overlay hinges and MDF doors, but it's a 90 gallon as opposed to this client's 200+ gal. tank.

Hope this helps...