The bottom side frame pieces are laid flat and the corners are cut at 45 degrees and secured with either 16 penny nails or screws. Here is an enlarged view of the bottom frame rail assembly showing the end cuts and attaching hardware. The top frame is the same, except that the base plate 2 X 4's are set on edge.
The side pieces and tank support pieces of the top of the frame are on their edges and also cut at 45 degrees and secured with 16 penny nails or screws. Here is an enlarged view of the interior tank support cross frame members and how they are arranged on the top of the vertical supports and attached to the side rails. If you wish to install doors in the front of the cabinet, you can eliminate the center vertical support member and double the front horizontal member.
It is recommended that a piece of 1/2" piece of plywood be installed on the top of the frame, with a 1/2" piece of sheet styrofoam on top of that. This will give solid support to the tank, distributing the weight evenly.
A plywood floor can be installed in the frame, making cut outs for the vertical 2 X 4 supports and installing the plywood on top of the frame base plates. It is recommended that 2 X 4 cross base plates (laid flat) be installed between the base side rail plates in the same manner as the top tank support cross members if a cabinet floor is to be installed.