The two dark lines are 1x2's you put these around the frame.
Without this you wouldn't be able to screw the 1x6 boards in place. This will also may the the part the stand sits flesh with the 1x6 panels
In the picture below, with the panels flesh to the frame of which the tank sits on. you add a board for trim on the top an bottom to hide the screws or nails you used to put the 1x6 on with. You can see the trim on top and bottom. This also makes it look like the tank is in the stand too. Its a nice look.
In the picture below in the corner you can see a game between the 1x6 and the leg post. It's between the top and bottom 1x2 boards. In every corner you can cut and measure 8 more 1x6 and slide those in behind The the structure legs.