I would attempt to move the column to directly under the corner. Then I would make 2 sheets of plywood that cover the tank. Use deck screws for when you need to pull off a panel for access. I am guessing that is once per year, but maybe I am wrong. Using thick plywood 3/4" coming to mind, you could probably hang a lot of stuff from that surface. Even pegboard could be laminated with 1x spacer blocks.