Test fit the wood pieces together and then pre-drill the holes for the screws. Apply Elmer's Wood Glue to all the contact points, and screw the pieces together. CAUTION : Make sure you are very accurate. If the beams are not assembled with precision, you will end up with a base or top that is not straight and may cause the tank to warp or even leak!!!

At this point I made the base frame and the top frame, which are identical.