If the bottom horizontal panel on the stand sits directly on the floor (its hard to tell from the picture, it almost looks like it might have spacers or legs of some kind elevating it a bit) I think the easiest way would be just double up the thickness of the vertical panels. There is a lip beyond the end panels where the top and bottom horizontal panels extend a bit. I would cut identically sized panels out of 3/4 inch plywood with exterior glue (for water resistance) and glue/screw them right to the end panels between the lips on the top and bottom panels. You can do the same thing with the verticals between the ends. Can't tell if there is a shelf behind the door, if not you can add full length panels inside that space. If there is a shelf you would have to add a panel below the shelf and another above directly over the bottom panel with the shelf between. Again, it's a little hard to tell from the picture but it would be best if these panels were the wide enough so that both the front and back of the aquarium frame were supported. You could also double up the top horizontal panel if you wanted to beef that up. If there is a space between the floor and the bottom panel you should have a support between them directly under any of the vertical panels.
Last edited by jr125; 02-22-2016 at 02:56 PM.