Proper wood screws have an unthreaded portion on the shank which is designed to turn freely in the piece closest to the head, while the threaded part engages in the other workpiece. This causes the workpieces to be pulled tightly together as the screw is tightened. Drywall screws are threaded all the way up, causing the threads to engage on both workpieces which can prevent them from being pulled tightly together or "jack" apart as it is tightened. Drywall screws are also tempered, to make them harder and thus resistant to stripping when used with screw guns. This makes them more brittle and prone to breakage when used for a structural application. For assembling 2x4's for aquarium stands, you should really use deck screws.