Lay two of your 30-inch boards on your workspace so they are parallel, on edge and 9 inches apart. Drive a screw that is ¾-inches away from the end of one of the boards into a 9-inch board. The 9-inch board should be perpendicular to the 30-inch board and on edge as well. Repeat this process at each end of the two 30-inch boards, so you have a 30-inch-by-12-inch frame. There should be 12 screws used for this frame. Build another 30-by-12 frame with your last two 30-inch and 9-inch boards.
Lay one of your frames flat on your workspace. Place a 31-inch board in each corner of the frame so they are standing up, perpendicular to the frame. Screw through the frame into the 31-inch boards. These boards are the legs for your table.
Lift your other 30-by-12 frame over your table legs and screw it to the legs so the frame is parallel with the first, and 16 inches directly above it. Lift your table frame so it is standing on its legs, and so the frame that you attached first is on top.
Screw your 30-inch-by-12-inch plywood to the top of your table frame to finish the top of the stand. Screw your 20-inch-by-12-inch plywood to the lower frame to finish the shelf that will be underneath your aquarium.
Position your aquarium stand where you will prefer your tank to be. Set your tank on top of the stand and then proceed to fill with water.